can ecg on Apple Watch detect PVCs?

I don't believe so, no.

• The ECG app cannot detect a heart attack. If you ever experience chest pain, pressure, tightness, or what you think is a heart attack, call emergency services immediately. 
• The ECG app cannot detect blood clots or a stroke.
• The ECG app cannot detect other heart-related conditions. These include high blood pressure, congestive heart failure, high cholesterol, or other forms of arrhythmia. 
• If you’re not feeling well or are feeling any symptoms, talk to your doctor or seek immediate medical attention. 

For more info about the ECG app, go here: https://support.apple.com/en-us/HT208955#know