On a Lenovo SR630 V4 it took Power/Performance Bias set to OS Controlled, and on an HPE DL380 Gen 11 it was the iLO Power Regulator moved off Dynamic Power Savings to OS Control Mode, with a host reboot.
Both then returned real values from xenpm get-cpufreq-para instead of the failure line.
@bleader is right that two vendors is not enough to document, so if your host works, could you post the vendor, the exact setting name, where it lives, and whether you needed a reboot? Dell, Supermicro and anything AMD are the obvious gaps.
I don't know whether the firmware is withholding p-state control outright or just not publishing the ACPI objects Xen looks for, so I would rather collect settings than write up a mechanism I can't back.
Thanks!