I got Win XP running under Xen on a Intel Dual Core CPU which supports VT technology. I am using FC6. Fedora has a script for creating domains and a GUI for managing the domains. It works very well. The one show stopper is that during the Win XP install you have to press F5 when the screen comes up saying "Press F6 for additional drivers". After pressing F5 you select "Standard PC..."
-- Bill