I'm currently using a KVM virtual machine as my primary file/media server. Since I have been using a virtual machine as my file server I have witnessed strange stalls in media playback when accessing media files via a NFS share. Media files would take a couple of seconds to load and occasionally playback would stall for anything from a second to 20 seconds. In addition scp transfers would also occasionally stall. With not much to go on I turned to Google which did turn up an old Redhat bug report from 20091. One of the solutions was to disable TCP offload in the KVM guest via:
mfs@wvm1 # ethtool -K eth0 tx off
Even though this was an old bug report, once I did this the stalls went away. Media playback is perfect. I haven't tried any scp transfers though I am confident they will now complete without stalling as well. For the time being I will be turning off TCP offload on all of my KVM guests.