Swagger 2.0 vs OAS3.0

pet_store_2_0.yml

swagger: '2.0'
info:
  description: This is a sample server Petstore server. 
  version: 1.0.0
  title: Swagger Petstore
  
host: virtserver.swaggerhub.com
basePath: /nazarhussain/Swagger2/1.0.0

schemes:
 - https
 - http
 
paths:
  /pet:
    post:
      summary: Add a new pet to the store
      operationId: addPet
      consumes:
      - application/json
      - application/xml
      produces:
      - application/json
      - application/xml
      parameters:
      - in: body
        name: body
        description: Pet object that needs to be added to the store
        required: true
        schema:
          $ref: '#/definitions/Pet'
      responses:
        405:
          description: Invalid input
      security:
      - petstore_auth:
        - write:pets
        - read:pets
  /pet/{petId}:
    get:
      tags:
      - pet
      summary: Find pet by ID
      description: Returns a single pet
      operationId: getPetById
      produces:
      - application/json
      - application/xml
      parameters:
      - name: petId
        in: path
        description: ID of pet to return
        required: true
        type: integer
        format: int64
      responses:
        200:
          description: successful operation
          schema:
            $ref: '#/definitions/Pet'
        400:
          description: Invalid ID supplied
        404:
          description: Pet not found
      security:
      - api_key: []
securityDefinitions:
  petstore_auth:
    type: oauth2
    authorizationUrl: http://petstore.swagger.io/oauth/dialog
    flow: implicit
    scopes:
      write:pets: modify pets in your account
      read:pets: read your pets
  api_key:
    type: apiKey
    name: api_key
    in: header
definitions:
  Category:
    type: object
    properties:
      id:
        type: integer
        format: int64
      name:
        type: string
    xml:
      name: Category
  User:
    type: object
    properties:
      id:
        type: integer
        format: int64
      username:
        type: string
      firstName:
        type: string
      lastName:
        type: string
      email:
        type: string
      password:
        type: string
      phone:
        type: string
      userStatus:
        type: integer
        format: int32
        description: User Status
    xml:
      name: User
  Tag:
    type: object
    properties:
      id:
        type: integer
        format: int64
      name:
        type: string
    xml:
      name: Tag
  Pet:
    type: object
    required:
    - name
    - photoUrls
    properties:
      id:
        type: integer
        format: int64
      category:
        $ref: '#/definitions/Category'
      name:
        type: string
        example: doggie
      photoUrls:
        type: array
        xml:
          name: photoUrl
          wrapped: true
        items:
          type: string
      tags:
        type: array
        xml:
          name: tag
          wrapped: true
        items:
          $ref: '#/definitions/Tag'
      status:
        type: string
        description: pet status in the store
        enum:
        - available
        - pending
        - sold
    xml:
      name: Pet
  ApiResponse:
    type: object
    properties:
      code:
        type: integer
        format: int32
      type:
        type: string
      message:
        type: string
externalDocs:
  description: Find out more about Swagger
  url: http://swagger.io

pet_store_3_0.yml

openapi: 3.0.0
info:
  description: This is a sample server Petstore server.
  version: "1.0.0"
  title: Swagger Petstore

servers: 
   - url: virtserver.swaggerhub.com/nazarhussain/Swagger2/1.0.0
     description: Sweagger Hub Server 
     
paths:
  /pet:
    post:
      summary: Add a new pet to the store
      operationId: addPet
      responses:
        '405':
          description: Invalid input
      security:
        - petstore_auth:
            - 'write:pets'
            - 'read:pets'
      requestBody:
        $ref: '#/components/requestBodies/Pet'
  '/pet/{petId}':
    get:
      tags:
        - pet
      summary: Find pet by ID
      description: Returns a single pet
      operationId: getPetById
      parameters:
        - name: petId
          in: path
          description: ID of pet to return
          required: true
          schema:
            type: integer
            format: int64
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
            application/xml:
              schema:
                $ref: '#/components/schemas/Pet'
        '400':
          description: Invalid ID supplied
        '404':
          description: Pet not found
      security:
        - api_key: []

components:
  schemas:
    Category:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
      xml:
        name: Category
    User:
      type: object
      properties:
        id:
          type: integer
          format: int64
        username:
          type: string
        firstName:
          type: string
        lastName:
          type: string
        email:
          type: string
        password:
          type: string
        phone:
          type: string
        userStatus:
          type: integer
          format: int32
          description: User Status
      xml:
        name: User
    Tag:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
      xml:
        name: Tag
    Pet:
      type: object
      required:
        - name
        - photoUrls
      properties:
        id:
          type: integer
          format: int64
        category:
          $ref: '#/components/schemas/Category'
        name:
          type: string
          example: doggie
        photoUrls:
          type: array
          xml:
            name: photoUrl
            wrapped: true
          items:
            type: string
        tags:
          type: array
          xml:
            name: tag
            wrapped: true
          items:
            $ref: '#/components/schemas/Tag'
        status:
          type: string
          description: pet status in the store
          enum:
            - available
            - pending
            - sold
      xml:
        name: Pet
    ApiResponse:
      type: object
      properties:
        code:
          type: integer
          format: int32
        type:
          type: string
        message:
          type: string
  requestBodies:
    Pet:
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Pet'
        application/xml:
          schema:
            $ref: '#/components/schemas/Pet'
      description: Pet object that needs to be added to the store
      required: true
    UserArray:
      content:
        application/json:
          schema:
            type: array
            items:
              $ref: '#/components/schemas/User'
      description: List of user object
      required: true
  securitySchemes:
    petstore_auth:
      type: oauth2
      flows:
        implicit:
          authorizationUrl: 'http://petstore.swagger.io/oauth/dialog'
          scopes:
            'write:pets': modify pets in your account
            'read:pets': read your pets
    api_key:
      type: apiKey
      name: api_key
      in: header

externalDocs:
  description: Find out more about Swagger
  url: 'http://swagger.io'