Hi,
I am having problems with KVM + pfsense 2.1, too.
I updates my Proxmox VE with:
apt-get update
apt-get dist-upgrade
then I did:
wget http://download.proxmox.com/debian/dists/squeeze/pvetest/binary-amd64/pve-qemu-kvm_1.4-7_amd64.deb
dpkg -i pve-qemu-kvm_1.4-7_amd64.deb
rebootet...