What you're after is called an "escutcheon", or "escutcheon plate". It's basically a decorative plate that goes under or around any hardware to cover a hole or provide support.
If you have screws holding the knobs on your cabinet doors, you might want to put something like this under each knob:
There are obviously many different styles of escutcheons, and this one might not suit your cabinetry.
Maybe go to Rockler or Amerock's web sites and phone their 1-800 customer service numbers and ask what they have for escutcheons for cabinet door knobs. I know you can buy cabinet door knobs with small escutcheons already molded into them. You might want to combine those kinds of knobs with larger escutcheons to protect a much larger area of paint around the knob.
I think if you just phone up Rockler or Amerock and say you're looking for an escutcheon to fit under a cabinet door knob, they'll refer you to a page in their online catalogue.