I have never used Xen but I use Qemu/KVM via virtmanager on a Debian 10
host. I have installed several UEFI (OVMF) booting guest OSes,
including Debian with no particular problem.
The problem of your Debian guest not booting could be caused by the
Debian installer not being run in UEFI mode, the media upon Debian is to
be installed being not GPT type, the mandatory EFI partition being
absent, the GRUB bootloader being not installed or incorrectly set-up,
and so on...
Debian installation guide (UEFI and general):
https://www.debian.org/releases/stable/amd64/ch03s06.en.html#UEFI
https://www.debian.org/releases/stable/amd64/index.en.html
There is an OVMF page on the Xen wiki, detailing setup and troubleshooting:
https://wiki.xenproject.org/wiki/OVMF
