Unfortunately I'm not that familiar with Xen to tell you how-to configure it to run a hypervisor under it, not even sure if it even works and if it works what kind of performance you get (I suspect very slow) and what limitations will be. It certainly will be highly experimental.
That's a xen question more as a vmware one. I did a quick search and found that xen has "some" support for nested virtualisation:
If you cannot rebuild the host, then I would still recommend to switch to another non-xen kernel.
If you can't reinstall your host, then you're probably out of luck, because you're running a 32-bit Xen hypervisor, which does not appear to virtualize long mode.
What an problem I'm facing the same error how can I just avoid it?
I'm on exagear emulator and my cpu is 64bit as well