As a result of a Stackoverflow question on the Tanenbaum/Torvalds debate I got to see a certain number of people claim that (virtualization) hypervisors are more or less microkernels, certainly enough so to make one of Tanenbaum's predictions (ultimately) correct. This is wrong. It's true that hypervisors and microkernels can both sit 'underneath' normal monolithic kernels as a low-level layer, and both can potentially be 'small' for some vague definition of small. But those are about the only solid points of similarity, and once you look at their purposes and how they are supposed to be used they become very different.

Microkernels are intended to create a minimal set of low-level operations that would be used to build an operating system. While it's popular to slap a monolithic kernel on top of your microkernel, this is not how microkernel based OSes are supposed to be; a real microkernel OS should have lots of separate pieces that used the microkernel services to work with each other. Using a microkernel as not much more than an overgrown MMU and task switching abstraction layer for someone's monolithic kernel is a cheap hack driven by the needs of academic research, not how they are supposed to be.

(There have been a few real microkernel OSes, such as QNX; Tanenbaum's Minix is or was one as well.)

By contrast, hypervisors virtualize and emulate hardware at various levels of abstraction. This involves providing some of the same things that microkernels do (eg memory isolation, scheduling), but people interact with hypervisors in very different ways than they interact with microkernels. Even with 'cooperative' hypervisors, where the guest OSes must be guest-aware and make explicit calls to the hypervisor, the guests are far more independent, self-contained, and isolated than they would be in a microkernel. With typical 'hardware emulating' hypervisors this is even more extremely so because much or all of the interaction with the hypervisor is indirect, done by manipulating emulated hardware and then having the hypervisor reverse engineer your manipulations. As a consequence, something like guest to guest communication delays are likely to be several orders of magnitude worse than IPC between processes in a microkernel.

Or in short: writing a microkernel style OS on top of a hypervisor is pretty clearly an absurd notion. This shows that hypervisors are not a species of microkernel.

(I'm sure this is plenty obvious to most people as it is, but I feel like writing it down here in order to be explicit.)