This is a basic XML editor that shows of how to do very simple XML validation inside of a PyQT QTextEdit field. All this does is check for document well-formed-ness, but using the LXML library it shouldn't be difficult to extend this to perform Schema or DOCTYPE based validation.

pyqt_xml_editor.py

import sys
from lxml import etree
from PyQt4 import QtCore, QtGui

class editor(QtGui.QMainWindow):
    def __init__(self):
        super(editor, self).__init__()
        self.text = QtGui.QTextEdit()
        self.setCentralWidget(self.text)
        self.text.textChanged.connect(self.validate)
        
        self.statusBar()
        self.setWindowTitle('Simple XML editor')
        self.show() 
    def validate(self):
        try:
            root = etree.fromstring(str(self.text.toPlainText()))
            self.statusBar().showMessage("Valid XML")
        except etree.XMLSyntaxError, e:
            msg = e.error_log.last_error.message
            line=e.error_log.last_error.line
            col=e.error_log.last_error.column
            self.statusBar().showMessage("Invalid XML: Line %s, Col %s: %s"%(line,col,msg))
        except:
            self.statusBar().showMessage("Invalid XML: Unknown error")
        
def main():
    app = QtGui.QApplication(sys.argv)
    w = editor()
    sys.exit(app.exec_())
 
if __name__ == "__main__":
    main()