I am a newbie to the world of photography.All these days i was using a point & shoot camera (Nikon Coolpix L820) and now i want to buy a compact system camera. So can anyone suggest which one will be best for me- Olympus PEN E-PM2 or Olympus PEN Lite E-PL3 or Nikon 1 J1 and why ?
The key to choosing between anything is understanding differences and evaluating their importance to your needs. There is no way we can tell you which is best for you because we have no idea what subjects you photograph, under what conditions and what medium you plan on using. I covered the differences and reasons for all mirrorless systems here which should help you choose.
One of the most basic things is to consider the balance between sensor-size (where larger means more quality) and gear size (where smaller is often more convenient). Given examples you list, both Olympus use sensors which is larger than that of the J1, which gives them superior image-quality. Here is what the Nikon 1 J3 looks like next to the Olmypus E-PM2.
Image-quality is not all though and you have to look at features as well. One distingishing aspect of the Nikon 1 system is their on-sensor Phase-Detect AF which is much faster than the Contrast-Detect AF used by Olympus. Not that the latest PENs are slow but there is a noticeable difference. The J-series also offer all-electronic shutter which makes them ultra-quiet. As a matter, these are the most quiet of any cameras which I've seen.