This is easily explainable. If you boot into DOS, you will also see the temperature rise. Why? Because back in the days that DOS and BIOSes were first written, overheating was not really a problem, so there was no need to idle the CPU. (it is somewhat lazy because one would think that BIOS developers would be writing them properly these days, but I suppose that it is hard to justify re-writing the whole thing from scratch when they can just modify the existing one.)
What this means is that the BIOS (basically just a program), like programs running DOS mode, just runs in an infinite loop to accept user input. Again, this was not a problem way back when, but with more modern systems, it can cause the CPU to heat up unnecessarily.
If you run DOS in a virtual machine, the CPU of the host system will shoot up to ~100% (at least for the core the VM is running on if there are more than one). That is why you are advised to run a little program called idle.com in your DOS VMs. It first came with VirtualPC, but has since spread for use by anyone using DOS in a VM. In fact, some even use it to keep their CPU idle while booting into actual DOS (though whether that works remains open to debate until someone can get around to disassembling it to see how it works—hmm, new project!)
- quoted from superuser.com