[libvirt] List of available libvirt drivers installed?

I am wondering if there is a way to show a list of available libvirt hypervisor drvers installed in an existing libvirt installation. I know based on the version of libvirt, I can find out supported drivers from this matrix: http://libvirt.org/hvsupport.html However, the particular installation may not turn on all available drivers for this particular build. Is there a way to find out? Particularly, I want to find out if the default libvirt coming with RHEL-5/6 supports VMware ESX, and the same for Ubuntu-10.4/10.10. Thanks in advance. Shi -- Shi Jin, PhD