There’s a short summary in this article:
IFL Science: Pavlov’s Dog Experiment Was Much More Disturbing Than You Think

I’m not certain on the accuracy of everything it’s saying, but other sources at least seem to agree he was originally studying digestion, and cut holes in the dogs’ throats so the food would fall out when they ate.

Here’s a video that goes into it

From memory he did throw them into boiling water.