[libvirt] Sharing file between host and guest

Hi All, Question not very related to libvirt but I think most of the real VM experts are in this list. What is the best and fastest way to share files between guest and host? Something like the share folders-hgfs in vmware might work, but is there a way that works on all hypervisors? Network based file sharing (NFS/Samba) are limited by the speed of the virtual network interfaces. Maybe one can do it through USB? Thanks a lot! -yushu