Hi, Poovaraj. None of these questions are directly related to REST-assured.
People use a few different toolkits and approaches for serializing and deserializing XML with Java. Two of the most popular ones are JAXB and Jackson. Try searching Google for "xml to pojo" and choose the toolkit that seems best for you, if that's really what you want.
It's not really necessary to deserialize XML into an object for testing most of the time. You can test the XML payload of an HTTP response using the tools built right into REST-assured. See here for more:
Best wishes,
Todd.