What Kernel are you using? For xen.efi full support, make sure you're using the latest one, 3.17. With previous Kernels, Xen UEFI boot also blackscreened on my computer after loading the Hypervisor. Seems that it freezed during Dom0 loading. Kernel 3.17 adds official UEFI Dom0 support: www.phoronix.com/scan.php?page=news_item&px=MTc1NDg