I have pvcs that happen only when I have a fast heartrate. Drs says they are benign. But I have read benign happen at rest only. Which is true?

What you read -- Generally, what you read is true -- PVC at rest is much more benign than PVC with exercise. Please see a cardiologist if this continues --.
Your doctor. Premature ventricular contractions[pvcs] can happen at rest or with activity. Many of my patients notice them when they are quiet or slowing down at the end of the day, even though the happen at other times as well. They are almost always benign--which means harmless. As always your personal provider is the best person to discuss your concerns or questions.