Thanks for the response..
I found the answer.
javax.faces.viewState maintains only the component tree with its values.
Example. if a page has a an input text and the value(Field Name="test") user enters for that text is 10, it stores this value.
When the scope of hte managed bean set to session, the values stored in httpSession.
If you try to change the value of "test" in MB to 15, on the response postBack we get only 15. Which implies that the value is restored from MB.