A common misconception - OS X is UNIX.

[Tim]

99% of people think that OS X was built from scratch and that ALL of the code was done internally by Apple; this is NOT the case - OS X is actually based on BSD UNIX;

Yeah, to run OSX under a VM you have to run it in a BSB configuration (and a few other things). I doubt that much is common though appart from maybe parts of the kernel.