QEMU/KVM can be invoked directly without libvirt, however you cannot to use tools such as `virt-manager`, `virt-install`, or `virsh`. Plain QEMU (without KVM) can also virtualize other processor architectures like ARM or PowerPC.
Fedora can run as a Xen guest operating system and also be used as a Xen host (with the latter being true from Fedora 16; for using an earlier version of Fedora as a Xen host, check out the experimental repo available at http://myoung.fedorapeople.org/dom0). For a guide on how to install and setup a Fedora Xen host, see http://wiki.xen.org/wiki/Fedora_Host_Installation[Fedora Host Installation] page on the Xen Project wiki.
OpenStack consists of a number of services for running infrastructure as a service (IaaS) clouds. They are the Object Store (Swift), Compute (Nova), and Image (Glance) services.