We look to continue running some apps on Dom0/KVN host, and I like to know, if there any performance difference between Xen/KVM enabled kernel, and "clean" regular kernel.
Have someone ever bench-marked stuff as this one?
I don't have any benchmarks but we run some very large and heavily used databases (PostgreSQL) on Xen dom0 so that in a pinch we could run some domU's if needed. We've never noticed any performance degradation compared to stock kernel.
I haven't tested KVM yet, but I've done a bit with Xen.
There is very little (if any) cpu performance drop. And for heavy workloads you may notice a drop in disk or network I/O.
One surprising result we had: after converting a physical server running a heavy Java workload (Teamcity continuous integration cycles), to a Xen domU the time taken to run all the builds and tests was roughly halved. Both machines were running Debian Etch.
The physical machine had 4 cores @ 1.6GHz, and the virtual machine had 4 cores @ 2.0GHz and both had the same disk subsystem and motherboard/ram etc, so I expected a toss-up between slightly faster cpu on one side and a slight drop in performance due to virtualisation on the other.
I was surprised how much faster the Xen domU actually was.
But I don't think that is very typical. In most other cases, I haven't noticed any obvious performance drops with paravirtualised domUs. Windows HVM domUs are a different story though - especially when SMP was involved (last time I looked).