daemon and clients to manage virtual machines and containers
The libvirt project provides a common interface to manage many different virtual machine and containerization products. Around a dozen different ones are supported, including QEMU/KVM, VirtualBox, Xen, and LXC.
Libvirt can also manage network and storage configuration. It supports several host operating systems, including Linux, FreeBSD, and Mac OS X (Windows is supported too, but those questions generally belong elsewhere on the Stack Exchange network).
Libvirt includes a daemon (libvirtd), API, and command-line client (virsh).
There are other programs that use libvirt; many have their own tags.
