It's not true that installing Hyper-V causes power management to be disabled. Instead, when Hyper-V is installed, the OS on your physical computer, called "parent partition", is just another virtual machine. This is different from how the old Virtual PC/Virtual Server worked.
So Task Manager shows your virtual CPUs, not physical CPUs. And for virtual CPUs, it can only report what the hypervisor allows it to see. E.g. it will show very low CPU load even if you have virtual machines running with very high load. Another effect is that it cannot see the actual speed.
I have several servers running Hyper-V and I can confirm via CPU-Z that core speed goes down when there is less workload, and it goes up when there is a lot of load.
I have also enabled Hyper-V on my laptop running Win8.1, and even though Task Manager always shows 2.89 GHz for the i7-3520M CPU, I can use the machine for ~4-5 hours on battery, with normal load, just like without Hyper-V being enabled. And of course CPU-Z shows much lower core speed for most of the time (or even up to 3.5 GHz when pushed hard).
Maybe bad drivers might affect this negatively, however it worked on any machine I tested so far without messing with drivers or configuration (Lenovo Thinkpad laptops and Supermicro servers with Xeon E2xxx, E5-xxxx).