I have done a temporary build of 4.2.0-rc2 for testing which is at http://koji.fedoraproject.org/koji/taskinfo?taskID=4386920 This also adds the efi hypervisor back in for x86_64.
Note to other people: fedora kernels still don't have xen dom0 EFI patches included, mainly because dom0 EFI patches are not yet ported to pvops or merged to upstream Linux.
So actually testing xen.efi hypervisor on UEFI systems is a bit tricky atm.. (you need to manually build a kernel based on the suse xenlinux sles11sp2 or opensuse patches).
I have tried the 4.2.0-rc packages on three machines: one with an AMD Athlon X2, one with an AMD FX 4170 [1], and a MacBook with a Core 2 Duo.
The first machine works fine.
The FX 4170 and MacBook are unable to load Dom0. I see Xen print its boot messages, but the machines spontaneously reboot before Dom0 prints any output.
[1] Same machine referenced at https://bugzilla.redhat.com/show_bug.cgi?id=841330