Why would you want qemu running in UEFI, rather than have qemu booted from UEFI? (more specifically run qemu on xen booted from UEFI, I think is what you'd want).

Practically speaking, QEMU needs a lot of OS functionality, such as scheduling, threading, virtual memory, and network, disk, and filesystem drivers, to be able to function. Xen does have these, but so does Linux, and KVM just exposes hardware virtualization as a module from Linux.