Hello, not sure I can actually help, but I'll try to bring some info.
As far as I know, Xen handles the cpu scaling by default, as you checked with xenpm. Therefore I would guess it does not expose the p-states to the guest. When reading /proc/cpuinfo the kernel will call show_cpuinfo() function will call arch_freq_get_on_cpu() which ends up checking support for p-states on fallback to a simpler version that will in our case probably always report the base clock. At least that's my understanding, I went through this quickly and may have missed bits, but sound "logical" to me.
I think the guest tools won't help you there, they report information from host to guest only, and as mentionned in the first point the guest frequency reporting is not live frequency.
The only way I see how to confirm your VM is using turbo clocks would be to pin your VM vcpus to fixed pcpus, load the VM and check these pcpu with xenpm as you were doing.
It is interesting to me you're seeing turbo clocks there at all, from my tests for some other stuff, on epyc I actually never saw the clock go above the base clock, did you do anything specific to enable it? I guess BIOS setup to enable turbo, and maybe enable it with xenpm?