It isn’t really as if the operating system ceases to exist. More to the point, it ceases to be a separate entity from the compiled application. A unikernel, therefore, is an indivisible unit of computing logic. As a microservice, it carries the promise of unlimited scalability. And as a virtual machine, it is designed to run under the Xen Type 1 (hardware-level) hypervisor.

Source: Docker’s Unikernel Purchase and the Changing Role of the OS