I wouldn't really call OSv a unikernel. The main distinction is that in unikernel operating systems, the application IS the kernel. The OS is not an emulation layer, but the application itself. Application code is linked directly with the kernel to produce an operating system whose purpose is to execute a single application. One example is Mirage[0].
The big difference is that while OSv may be targeting the same use case, it's still more similar to a general purpose OS. Some of the kernel APIs it provides seem to be blurring the line though.