The Xen 4.0 domain 0 where all of the hardware drivers run defaults to the Linux 2.6.31 kernel, which supports the new processors coming out from Intel and Advanced Micro Devices, but you can get out on the bleeding edge with the Linux 2.6.32 kernel or step back to the 2.6.18 kernel if you like to be back closer to the pommel. Xen 4.0 also supports fault tolerance through a project called Remus, which is being created at the University of British Columbia and which was at its 0.9 release level back in November when Xen 3.4.2 came out.