The extra features in Xara Xtreme, not in Xara XS are
- Shadow, Bevel, Contour, Feather and Live Effect tools
- The Xara Picture Editor is included by can't accept Photoshop plug-ins
- No PDF export.

Otherwise it's the same as Xara Xtreme, and is built form the same codebase (so is 100% file compatible).

I don't think we need another forum as the way it works is identical in all respects to Xara Xtreme (except for the missing features of course).