Anyway, I can confirm GLX (and OpenGL) works on XEN x86_64!
22.214.171.124 - xen-unstable-tree (20 May 2006)
chuck@debian64:~$ glxgears -printfps
33106 frames in 5.0 seconds = 6620.652 FPS
35707 frames in 5.0 seconds = 7141.383 FPS
but, I had some trouble to get it to compile.
e.g phys_to_machine was undeclared, some strange errors & warnings...