The new consoles are
compatible with PC-class programs. Existing PC games and programs should port fairly easily to console, if one counts a new OS interface and perhaps a need to optimize for multiple cores as "easy."
And programs written for a console in the PC style should port similarly easily to a PC. BUT...
The consoles have a fundamentally different architecture: They use APU-style common memory. In contrast, a PC with a separate video card has separate main memory and video memory, and simply does not have common memory. No such PC can be bought or built.
While the APU approach has pluses and minuses, the minus for compatibility is that best APU efficiency involves data in common memory being worked on "simultaneously" by CPU cores and GPU cores. That option simply is not available on a PC with a separate video card. The only PC architecture similar to a console is an APU without a separate video card. And available APU's are not as strong as those in the consoles.
As console programs change design for maximum console efficiency (to appeal to the most-profitable market), they may become difficult to port to a normal PC. Possibly we could see releases with both a new "console" engine, and the old "PC engine." But only the console engine will continue to develop, and that will only work with common memory.
got it from a post from neogaf