I personally never run it on a VM, but I believe they solved this issue:
https://redmine.pfsense.org/issues/8954
Other people say it also works with VT-d enabled, if supported.
https://software.intel.com/en-us/ar...ms-for-efficient-virtualization-of-io-devices
Don't know if that would apply to Xen (my Qotom runs XCP-NG). I only experimented with OpnSense a bit last year (overall it felt nicer than pfsense, where some of the stuff I wanted to test with it didn't even work properly - namely that DPI engine which's name I forgot).