I've described it by saying unikernels are constructed by library operating systems. ie, the unikernel is the artefact but the libOS is what you (as a dev) actually work with.
The first line of the Wikipedia page is written this way - https://en.wikipedia.org/wiki/Unikernel
OSv is a bit unusual architecturally, it is almost a very small OS that runs applications in a single address space. It is perhaps a hybrid architecture.