Xen requires hardware virtualization support for OSes that aren't otherwise Xen aware. The best you could do is something similar to vmware before hardware acceleration was available.
Which isn't to say it's impossible, as VMWare was doing just that on the N810 not too long ago, just hard.
I'm pretty sure that someone is going to freak out at the fruit company if they see this. I imagine it's never going to happen, for legal reason. If nothing else, nobody here is licensed to modify nor distribute modified copies of an Apple OS.
Someone is going to be kidnapped in the middle of the night in a stylish white van.
".. aliens from planet apple abducts a maemian !! .. "
I think I have seen XenDesktop for ARM Linux somewhere.