[ryao]

Neither of them use microkernels. They are monolithic kernels with loadable module support ("hybrid" kernels). The cores of both Windows NT and XNU were originally microkernels, but then they put all of the servers into the kernel address space to make them into the monolithic kernels that they are today.

[iforgot22]

XNU apparently stands for "X is not Unix." Well that's confusing.

[ryao]

It actually was a recursive acronym that stood for XNU is Not UNIX. Then at some point Apple got it UNIX certified as part of MacOS.