Unfortunately, and I really mean that, it is a violation of the OS X software license to virtualize any version of OS X Client on any hardware.
OS X Server Snow Leopard and later may be virtualized on Apple Hardware and support exists in both VMware Fusion, Parallels, VirtualBox for doing so. None of them will virtualize OS X client. There are hacks which will get you around their checks, but it is still a violation of your OS X client license.
I have seen OS X Client and server virtualized using VirtualBox on both Apple and non-Apple hardware, so it is doable.
The fact that you cannot virtualize OS X Client on even Apple Hardware is a major pain for anyone engaged in testing software to run on OS X. While I can on a single system - and in fact we use Macs for it - exercise a full range of Windows configurations, I can't do the same for OS X Client legally.