Why does my CPU utilization differ in Hardware Monitor and Task Manager?

Your CPU is hot as hell.

The temperature on all cores is over 100°C. It's dropping its frequency as low as it can to prevent damaging itself. HWiNFO says 798 MHz, Task Manager says 0.57 GHz - probably just because they're not sampling simultaneously, but it's not important right now. You have to fix your CPU cooler immediately, because apparently it's not working at all. This can lead to permanent CPU and/or motherboard damage.

Now, to answer your question, both programs are correct, but they're displaying different information.

HWiNFO is telling you that the CPU is utilized at 100% of the 0.8 GHz clock it's currently running at.

Task Manager says that it's utilized at 17% of its normal 3.4 GHz clock, which is roughly equivalent to 100% utilization at current 0.57 GHz clock.