Hello,
There's a bug in Xen hvmloader, when compiled with gcc 4.6, causing HVM guests crashing on startup.
Upstream xen.org fix is here: http://marc.info/?l=xen-devel&m=131117428320002 Fedora bugzilla entry: https://bugzilla.redhat.com/show_bug.cgi?id=723569
The bugfix patch should be added to both F15 and rawhide/F16 Xen rpms.
Thanks,
-- Pasi