EL5 virtio drivers

I have an EL5 virtual machine that was already running under KVM (on EL6 host) paravirtualized. When moving it to linode, the kernel boots, and loads the virtio_blk driver as seen on the console. But it can't see any virtio disks. Changing the linode vm to full virtualization lets the kernel boot (it mounts filesystem by LABEL instead of device name).

o Why would the VM run paravirtual on my KVM host, but not on linode?

o Is there an incompatible change to virtio between EL6 and whatever linode uses for their hosts?

o Can we find out what virtio protocol version that linode uses?

The long term solution, of course, is to migrate this VM to EL6 - as EL5 is nearing EOL. I have several EL6 vms on linode, so I know linode virtio is compatible with EL6.