What is virt-manager ?

Quote from the post above:
Xen is really a tale of two separate projects: xen.org and Citrix XenServer. The xen.org version is a group of packages that you install to an existing operating system to re-create that system as a Type 1 hypervisor with a Xen-aware Linux kernel, some libraries and other tools. One of those tools is the virt-manager, which is your graphical virtual machine management interface.
End quote.
So, One of those tools is the virt-manager, which is your graphical virtual machine management interface. That's not right. Setup Xen Hypervisor&tools along with kernel supporting Dom0 and Libvirt/Virtinst/Python-virtinst/Virt-viewer setup are two different things. You may have Xen Instance running nicely and completely broken
Xen API integrated into libvirt, affecting virsh,virt-install,virt-manager. The first example is Ubuntu 10.04 Server.
There was (or is) ,actually, one exception. It's OpenSolaris. Via
historical time frame libvirt.hg was integrated into xVM mercurial
gate and got built and installed along with Xen Hypervisor and Tools. Consequently. pkg install xvm-gui installed all stuff at a time.