where xml file of project area is located in rational team concert.

could you explain a little more please..  not quite sure what you want.

the project XML does not have any VALUES of the attributes.
in the followup extension, you can get the CURRENT values (they were saved just before the extension was called). but you do not have an XML  structure, just the attributes, their values, and the attribute type (object). from that you could construct an xml document.

you can get the project XML from the project area
you can see code to get the xml structure here
https://jazz.net/forum/questions/159700/is-it-possible-to-migrate-an-rtc-project-area-to-a-different-process-parent-programmatically
in the accepted answer
                    // get the process data
                    @SuppressWarnings("unchecked")
                    Map<object, Object=""> pd = ip.getProcessData();

                    // get the content Manager
                    IContentManager icm = repo.contentManager();
                    // get the process config content object
                    IContent processSource = (IContent) pd.get(keyname);
                    if (processSource != null)
                    {
                        System.out.println("have xml source");
                        saveXMLSource(ip, processSource, icm, sPm);
                    }