libvirt users - Aug 2010 - how to make consisten backups of guests without stopping them?

Hello. I'd like to backup kvm guests running from lvm partitions on the host
without as little downtime as possible, preferably without stopping them. Could
libvirt help me with this? Could anybody point me to some documentation? The
question is what happens to the filesystem if I snapshot it from the host. LVM
is supposed to tell the filesystem to make itself consistent but I don't
know
how that works if the filesystem is mounted inside the guest. 

thanks
mk