Tianocore / OVMF is integrated into Xen 4.4 release with essential patches upstreamed. However in 4.4 it is not built by default.
To build Xen with OVMF support, you would need to have --enable-ovmf when running configure.
One thing to have in mind is Xen supports both its own QEMU fork called qemu-traditional and upstream QEMU called qemu-xen. OVMF only supports the latter. Xen 4.4 has upstream QEMU configured for all HVM guests by default, so it is fine to not specify which QEMU to use in guest config file. But if you have already configured qemu-traditional for your guest you would need to delete / comment out that line.
A typical guest config file looks like below. Nothing fancy, just the "bios=" option is important.
builder = 'hvm' name = 's0-efi' vcpus = 2 memory = 1024 # memory = 6000 vif = ['bridge=xenbr0'] disk = [ '/data/s0-efi.qcow2,qcow2,xvda,w' ] # This is a disk image with EFI guest installed, you can also use live CD if you prefer on_crash = 'preserve' vnc=1 vnclisten='0.0.0.0' serial='pty' bios='ovmf' # if you don't have this line, seabios will be used