Since the original Xbox was running mostly off the shelf hardware, I'm not sure it needs an emulator (aside from whatever security/copy protection hardware).
But the 360/PS3 is going to be tough. Tougher than average, I'd say since those were both custom CPUs. Yes, there is some papers out there covering how they did their execution but that doesn't cover some of the weird stuff. Stuff like with the PS2 and original PS that took years to sort out.
Those of you who don't remember the Bleem! saga and the fact that Sony not only lawsuited them to death, but also make emulation even harder by changing the way their compilers did certain undocumented graphic blits and other memory tricks. This was why Bleem! had a specific target list of compatible games.
Still not sure that all of that was documented.