The Xbox One won't play any existing Xbox 360 games, said Microsoft's Marc Whitten in an interview with The Verge.
The lack of backward compatibility results from the new console's hardware architecture, which is a significant departure from that of the Xbox 360. "The system is based on a different core architecture, so back-compat doesn't really work from that perspective," said Whitten, corporate vice president of Xbox Live.
The Xbox 360 does support certain original Xbox games through software emulation — more than 450 titles, almost half the console's library — although backward compatibility can be spotty, depending on the game in question.
Sony's PlayStation 4 is in the same situation as the next Xbox — it won't natively support PlayStation 3 games, because the upcoming system's AMD-made "Jaguar" x86 processor is too different from the PS3's Cell processor. Sony hopes to make back catalog games available on the PS4 through streaming and emulation.