http://x86vmm.blogspot.com/2005/11/xen-and-redhat.html
Both present "virtual machines" to client code. For that matter, so does UNIX, although the UNIX virtual machine has some very complex virtual instructions, like fork(2), exec(2), dup(2), etc. You can crudely map most software platforms on a continuum of abstraction, with something like Python's implicit virtual machine (which is dynamically typed and bound late) at one extreme, and a bare-metal VMM whose interface is identical to that of the underlying hardware at the other.
Both paravirtual hypervisors and microkernels extend the underlying hardware, and they do so at a lower level of abstraction than what we call an OS. In practice, the hypervisors extensions feel more like hardware (they might include device models, virtual memory translation, and interrupt models), while the microkernel's would feel more like software (providing RPC mechanisms, security models, abstractions like "thread" and "process", etc.).
Even in practice the line is grey sometimes. L4 used to call itself a microkernel; now it calls itself a hypervisor.