We have installed after many issues Xenserver 7.1 in UEFI mode.
Unfortunately if we boot in headless mode (no monitor attached) it does not boot.
If we attach a monitor there is no display so we cannot debug or take any kind of screenshot, but we know that Xen didn't boot. It appears to be a grub issue.
Of course, with a display attached from the beginning we can boot without any problem.