Nested Virtualization on KVM

Hi,

does anyone have experience with Nested Virtualization by nesting Xen 4.2 in KVM?
I have KVM as Host-Hypervisor and XEN as Guest-Hypervisor and trying to install WinXP as an HVM-Guest which just worked, but the XP seems to be very slowly!

It is quite confusing that executing /xl dmesg /gives me the output that
SVM should be enabled (XEN) HVM : SVM enabled! But a look into
/proc/cpuinfo shows, that there is no svm-flag listed in there.

So may the bad performance is caused through that problem that SVM is not available and I am working with Binary Translation to handle sensitive instructions?

Cheers,
Mike