Clamps for cutting PVC or other circular objects
There are an array of things that will clamp a PVC pipe.   

The best thing for a bench is probably a vice clamp - probably need to put a thin rag on each side.   They also make circle clamps but you will need one for various sizes.   

Probably the most used plumbing tool is a chain vice.   Coupled with some rubber matting or rags you can clamp PVC tight.



I have cut a lot of PVC in my life and I have never felt the need to clamp it.  You should be using a mitre saw or you should just buy a PVC pipe cutter - I use both ways.