I use bootcamp to install XP.
It depends on a few things. In full screen mode, with XP not really doing anything funky, your fans should not kick in or if they do only lightly and briefly. Afterall, if you look at your system resource use, Parallels will only take up about 5% CPU if XP is resting, which is less than even Firefox or iTunes playing a video, which should not cause the fans to get loud.
I have noticed my fans can go crazy with Parallels if any of the following are happening
- XP screen saver comes on (any) and Parallels is in Windowed mode
- In coherence mode, if IE is running any Windows media or flash
- If XP is running an update
- If XP is running a real time virus scan
- If Windows defender is running a scan
- If any maintenance tasks are running