Different CPU Types for KVM/QEMU Virtualization

basicly, qemu64 is the more compatible option if you want to do live migration between amd and intel for example, or between differents generations of processors. host definition is the real cpu options mapping

↬ https://pve.proxmox.com/pipermail/pve-devel/2012-July/002994.html

↬ https://serverfault.com/a/404355

TL;DR: If you don't plan to migrate your vm, use host - otherwise use qemu64.