Specifically XNU is continuation of OSFMK, which was mix of 2.0 and 3.0 stuff, with built-in bsd server instead of separate process, and which was the basis for

Specifically XNU is continuation of OSFMK, which was mix of 2.0 and 3.0 stuff, with built-in bsd server instead of separate process, and which was the basis for OSF/1 (and thus also Tru64, and quite possibly common with WPOS - IBM at one point was involved partially in the OSF consortium)