When you create your VM do you remember to choose 'other' for OS type and qemu64 for CPU?
What version of proxmox do you use exactly? (from proxmox host: pveversion -v)
My Nas4Free VM (9.1.0.1 - Sandstorm/Revision 847 -> FreeBSD-9.1) runs without Problems with "kvm64(default)" CPU.