I said that for both statements:
It's not Xen fault if it doesn't work on top of KVM, but Xen accessing some virt features that aren't correctly exposed by KVM. I'm not blaming them, nested virt is a mess to properly code and test. Consider this as experimental everywhere.
I missed the "for me". But it's also vague, I don't know your constraints/what do you want. Also, I think I heard that some users made it work with Virtualbox, but it might also depends on your hardware. As I said, nested virt is a mess, and sometimes, some hardware works better than others (or even some VBox versions).
