Comment 4 for bug 1520979

My few cents here. Its unfortunate but its known and not simple to fix. The current multiboot protocol does not allow UEFI boot as things transition into a mode that shuts down certain services from the UEFI BIOS side. Which I think will cause the hypervisor not finding any memory since this is reported in a different way which is only available if the UEFI boot services were not stopped. Upstream was working on the problem on and off but I don't think there is a final solution yet.
There is a EFI executable added to /boot when installing the hypervisor but that would have to be loaded by the UEFI BIOS directly (not sure how passing command line arguments is supposed to be done exactly). It is not signed either, so secure boot would not work any way.
The best way forward IMO would be to have the needed fixes in multiboot2, so there is a way to boot via grub2 in UEFI mode. Still that has the secure boot topic unresolved.

Preventing installation of Xen when in UEFI mode is probably bad either (plus I am not sure it can be safely done). So maybe the best option is to clearly state in the release notes that Xen does currently not boot in UEFI mode.