The Clover XSD schema extracted from `Clover-for-Ant` distribution (https://confluence.atlassian.com/display/CLOVER/Clover-for-Ant).

clover.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified">
    <xs:element name="coverage">
        <xs:complexType>
            <xs:annotation>
                <xs:documentation>
                    Top-most element describing the coverage report. Contains a
                    project and a test project.
                </xs:documentation>
            </xs:annotation>
            <xs:sequence>
                <xs:element ref="project"/>
                <xs:element ref="testproject"/>
            </xs:sequence>
            <xs:attribute name="clover" use="required" type="xs:NMTOKEN"/>
            <xs:attribute name="generated" use="required" type="xs:integer"/>
        </xs:complexType>
    </xs:element>
    <xs:element name="project">
        <xs:annotation>
            <xs:documentation>
                Project metrics relating to non-test source.
                @name - project name (optional)
                @timestamp - seconds since UTC
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:sequence>
                <xs:element name="metrics" type="projectMetrics"/>
                <xs:element maxOccurs="unbounded" ref="package"/>
            </xs:sequence>
            <xs:attribute name="name"/>
            <xs:attribute name="timestamp" use="required" type="xs:integer"/>
        </xs:complexType>
    </xs:element>
    <xs:element name="testproject">
        <xs:annotation>
            <xs:documentation>
                Project metrics relating to test source.
                @name - project name (optional)
                @timestamp - seconds since UTC
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:sequence>
                <xs:element name="metrics" type="projectMetrics"/>
                <xs:element maxOccurs="unbounded" ref="package"/>
            </xs:sequence>
            <xs:attribute name="name"/>
            <xs:attribute name="timestamp" use="required" type="xs:integer"/>
        </xs:complexType>
    </xs:element>
    <xs:element name="package">
        <xs:annotation>
            <xs:documentation>
                Package metrics.
                @name - the.package.name
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:sequence>
                <xs:element name="metrics" type="packageMetrics"/>
                <xs:element maxOccurs="unbounded" ref="file"/>
            </xs:sequence>
            <xs:attribute name="name" use="required" type="xs:NCName"/>
        </xs:complexType>
    </xs:element>
    <xs:element name="file">
        <xs:annotation>
            <xs:documentation>
                File metrics.
                @name - the file name e.g. Foo.java or Bar.groovy
                @path - the filesystem-specific original path to the file e.g. c:\path\to\Bar.groovy
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:sequence>
                <xs:element name="metrics" type="fileMetrics"/>
                <xs:element maxOccurs="unbounded" ref="class"/>
                <xs:element minOccurs="0" maxOccurs="unbounded"
                            ref="line"/>
            </xs:sequence>
            <xs:attribute name="name" use="required" type="xs:NCName"/>
            <xs:attribute name="path" use="required"/>
        </xs:complexType>
    </xs:element>
    <xs:element name="class">
        <xs:annotation>
            <xs:documentation>
                Class metrics.
                @name - the unqualified class name
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:sequence>
                <xs:element name="metrics" type="classMetrics"/>
            </xs:sequence>
            <xs:attribute name="name" use="required">
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <!-- class name have letters, digits (but not as first character), dollars and underscores -->
                        <!-- extra: Clover names inner classes as "Outer.Inner" for java and "Outer$Inner" for groovy -->
                        <xs:pattern value="[\p{L}$_][\p{L}\p{Nd}$_.]+"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:attribute>
        </xs:complexType>
    </xs:element>
    <xs:element name="line">
        <xs:annotation>
            <xs:documentation>
                Line-specific information.
                @line - the line number
                @type - the type of syntactic construct - one of method|stmt|cond
                @complexity - only applicable if @type == 'method'; the cyclomatic complexity of the construct
                @count - only applicable if @type == 'stmt' or 'method'; the number of times the construct was executed
                @truecount - only applicable if @type == 'cond'; the number of times the true branch was executed
                @falsecount - only applicable if @type == 'cond'; the number of times the false branch was executed
                @signature - only applicable if @type == 'method'; the signature of the method
                @testduration - only applicable if @type == 'method' and the method was identified as a test method; the duration of the test
                @testsuccess - only applicable if @type == 'method' and the method was identified as a test method; true if the test passed, false otherwise
                @visibility - only applicable if @type == 'method'
            </xs:documentation>
        </xs:annotation>
        <xs:complexType>
            <xs:attribute name="num" use="required" type="xs:integer"/>
            <xs:attribute name="type" use="required" type="construct"/>
            <xs:attribute name="complexity" type="xs:integer"/>
            <xs:attribute name="count" type="xs:integer"/>
            <xs:attribute name="falsecount" type="xs:integer"/>
            <xs:attribute name="truecount" type="xs:integer"/>
            <xs:attribute name="signature" type="xs:string"/>
            <xs:attribute name="testduration" type="xs:decimal"/>
            <xs:attribute name="testsuccess" type="xs:boolean"/>
            <xs:attribute name="visibility" type="visibility"/>
        </xs:complexType>
    </xs:element>

    <xs:complexType name="classMetrics">
        <xs:annotation>
            <xs:documentation>
                Metrics information for projects/packages/files/classes.
                @complexity - the cyclomatic complexity
                @conditionals - the number of contained conditionals (2 * number of branches)
                @coveredconditionals - the number of contained conditionals (2 * number of branches) with coverage
                @elements - the number of contained statements, conditionals and methods
                @coveredelements - the number of contained statements, conditionals and methods with coverage
                @statements - the number of contained statements
                @coveredstatements - the number of contained statements with coverage
                @methods - the number of contained methods
                @coveredmethods - the number of contained methods with coverage
                @testduration - the total duration of all contained test methods
                @testfailures - the total number of test method failures
                @testpasses - the total number of test method passes
                @testruns - the total number of test methods run
            </xs:documentation>
        </xs:annotation>
        <xs:attribute name="complexity" use="required" type="xs:integer"/>
        <xs:attribute name="elements" use="required" type="xs:integer"/>
        <xs:attribute name="coveredelements" use="required" type="xs:integer"/>
        <xs:attribute name="conditionals" use="required" type="xs:integer"/>
        <xs:attribute name="coveredconditionals" use="required"
                      type="xs:integer"/>
        <xs:attribute name="statements" use="required" type="xs:integer"/>
        <xs:attribute name="coveredstatements" use="required"
                      type="xs:integer"/>
        <xs:attribute name="coveredmethods" use="required" type="xs:integer"/>
        <xs:attribute name="methods" use="required" type="xs:integer"/>

        <xs:attribute name="testduration" type="xs:decimal"/>
        <xs:attribute name="testfailures" type="xs:integer"/>
        <xs:attribute name="testpasses" type="xs:integer"/>
        <xs:attribute name="testruns" type="xs:integer"/>
    </xs:complexType>
    <xs:complexType name="fileMetrics">
        <xs:annotation>
            <xs:documentation>
                Metrics information for projects/packages/files.
                @classes - the total number of contained classes
                @loc - the total number of lines of code
                @ncloc - the total number of non-comment lines of code
            </xs:documentation>
        </xs:annotation>
        <xs:complexContent>
            <xs:extension base="classMetrics">
                <xs:attribute name="classes" type="xs:integer"/>
                <xs:attribute name="loc" type="xs:integer"/>
                <xs:attribute name="ncloc" type="xs:integer"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:complexType name="packageMetrics">
        <xs:annotation>
            <xs:documentation>
                Metrics information for projects/packages.
                @files - the total number of contained files
            </xs:documentation>
        </xs:annotation>
        <xs:complexContent>
            <xs:extension base="fileMetrics">
                <xs:attribute name="files" type="xs:integer"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:complexType name="projectMetrics">
        <xs:annotation>
            <xs:documentation>
                Metrics information for projects.
                @files - the total number of packages
            </xs:documentation>
        </xs:annotation>
        <xs:complexContent>
            <xs:extension base="packageMetrics">
                <xs:attribute name="packages" type="xs:integer"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>

    <xs:simpleType name="construct">
        <xs:restriction base="xs:string">
            <xs:enumeration value="method"/>
            <xs:enumeration value="stmt"/>
            <xs:enumeration value="cond"/>
        </xs:restriction>
    </xs:simpleType>

    <xs:simpleType name="visibility">
        <xs:restriction base="xs:string">
            <xs:enumeration value="private"/>
            <xs:enumeration value="protected"/>
            <xs:enumeration value="package"/>
            <xs:enumeration value="public"/>
        </xs:restriction>
    </xs:simpleType>
</xs:schema>