I know VMWare does it...

What is other virtualization software that allows you to boot off PXE?

Yes you can use PXE with both Xen and KVM. You'll probably need the docs & Google by your side :)

I wasn't sure if Xen had the support built in for it or not - do you have any links handy? :)

Personally I haven't tried it but you'll probably want to swap out the bootloader for pypxeboot, https://www.cs.tcd.ie/Stephen.Childs/pypxeboot/

Thanks for the link .. any other ways that anyone knows of to do it with Xen? Or any pages on how to do with KVM?