First off, I am not a physician so I am not up on specific treatments to these conditions but hopefully I can explain things a little. In general terms the heart has four chambers. 2 atria and 2 ventricles. The atria sit on top of the ventricles. The right atria receives blood from the body (low in oxygen). The right atria then pumps the blood into the right ventricle. The right ventricle will then pump the blood to the lungs where the blood picks up more oxygen. Then the blood goes to the left atria which pumps the bood to the left venticle. From the left venticle the blood is then pumped to the whole body delivering the oxygen.
These chambers are controlled by electical impulses and when everything is working correctly everything works in a paticular sequence. These electrical impulses originate in a specific area called the SA node. this causes both atria to contract pumping the blood to the venticles. As this is happening the electrical charge travels to the AV node which tells the ventricles to contract. In an average person this can happen 60-100 per minute.
In A-fib the electrical signal does not start where it is suppose to (the SA node). These The problem with this is the atria can no longer pump the blood properlly to the venticles and and the sequence of contactions between the atria and venticles causes for a very inefficient pump. Blood then can pool in the atria and cause blood clots that can eventually travel to other parts of the body causing blood clots. electrical signals then don't travel properly and cause the atria to contract abnormally and out of rhythm.
PVC is Premature Ventricle contraction. Here the ventricle is contracting out of sequence (before the normal heartbeat).
In general A-fib is more serious but if PVC is severe or frequent enough needs treatment also.
I know I got long winded here but this is actually a generalization of what goes on but I hope this helps in understanding a little of what goes on.