Darwin is the base of Mac OS X, comprising the kernel “xnu”, the system libraries and the UNIX tools. Darwin does not include the Mac OS X GUI, and the future x86 version of Mac OS X will only run on Apple branded hardware. …at least the current builds of Mac OS X for x86 need SSE2 and SSE3 support of the CPU – the Xbox CPU has neither. And of course, OS X wouldn’t run with only 64 MB of RAM, and very slowly with only 128 MB. But here’s how to run Darwin on an Xbox… Link.
HOW TO run Darwin on Xbox