Found in xen wiki a remark of Jeremy Fitzhardinge about the performance of Xen 4.0 (Xen Summit at Intel Shanghai Nov 19-20, 2009, Slide 7):
Performance still needs attention
- Seems to be ~10-20% hit vs 2.6.18-xen
… but you get a current 2.6.31/32/… Linux kernel.
Preparations to boot a Xen 4.0 dom0 kernel: 2.6.18-to-2.6.31-and-higher
Status of Xen features in Linux kernel: XenParavirtOps