I've used pfSense in VirtualBox for years, as VPN-gateway VMs. I'm posting from a VM that hits the Internet through three VPNs in a nested chain. Each pfSense VM uses just ~70MB.

There's not an appliance per se but you can just install it to a virtual machine. I was running a 1 CPU, 256MB instance with a 100mbs line and noticed no issues - ESX running on Core i7-920

I really thought they had a blog post somewhere or maybe a tweet or something stating they were planning to support a vm, but I'm failing to find the link. Maybe I'm remembering incorrectly. In any case, they do have a forum section dedicated to virtualization:

https://forum.pfsense.org/index.php?board=37.0

pfSense works as a Xen guest.

I had the opposite experience, I had to disable all hardware offload to get it to work correctly on XenServer.

There was a bug in pf which made it break with TSO, but it was fixed a couple years ago.