It is that interfaces to hypervisor - it is not a hyperviser itself. Supports a wide range of hypervisors and directs placement of virtual machines by talking to hypervisor.
The default supported hypervisor is KVM, this is because OpenStack is very Linux oriented. But others are supported as well, QEMU, HyperV - also you can use VSphere a method to contact ESXi.
Typically the machines cannot be moved between hypervisors - you cannot have a hybrid cloud where a multitude of hypervisors are used.
Networking is no more included in Nova, it was in early days - not more relevant anymore as there is a separate component for that - Neutron.