Do we need some new terminology? There does not seem to be a word for the kind of thing Hurd is. "Replacement for the Unix kernel" seems a bit wordy. It's a thing that does what ordinary folk expect a kernel to do. So technically, it's a kind of virtual kernel , I suppose, but that would be far too easy to misunderstand. On a completely different note, just what is it about some software projects that stretches them…
Technically it's not any kind of kernel. Mach is the kernel, and Hurd is a collection of servers.