Relationship between openstack, KVM, qemu-KVM and libvirt

Openstack, KVM, qemu-KVMAndLibvirtRelationship:

KVM Is the lowest level Hypervisor , It is used to simulate CPU , It lacks Network And perimeter I/O So we cannot use it directly. QEMU-KVM Is a complete simulator, It is based on KVMThe above, It provides the complete network and I/O Supported .
Openstack Not directly controlled Qemu-KVM , It uses Libvit Library for indirect control Qemu-LVM ,
Libvirt Provide boast VM Functions of the Platform, It can control QemuSimulator, Including VMWare,
Virtualbox xen And so on. So Openstack Boast VM , So Openstack Only Libvirt Instead of directly using Qemu-KVM . Libvirt It also provides some advanced functions, For example Pool/vol Management.