We have multiple Centos and Windows VM's running in VMWare ESX.  I don't believe you would see any performance variance with a true VM unless you are somehow overtaxing the hardware elsewhere.  The main advantages are backups and data recovery.  If you have a hardware failure, just move the VM to another box, and it runs as before. 

Hi,

I was getting ready to move my mail server to new hardware and was
considering trying it in a VM.  I have tried it before in an OpenVZ
container, but had many problems.  Of course, this is not a real VM.
I have been migrating quite a few other applications to KVM  and have
experienced no problems at all.  I was wondering if anyone had advice
or an experience to share from stability, performance or any other
point of view.

Thanks.

Tony Zakula