There should not be any systematic control at the airport for flights between Paris and Athens. This means that your passport won't be stamped and you will probably not even see any border guard on the way to or from Paris by air (on the other hand, going by land through Serbia, Romania or Albania would mean leaving and reentering the Schengen area which your visa may or may not allow). Your time in France and Greece would be part of a single stay in the Schengen area, covered by your visa.
Beyond that, adding a short week-end trip to Paris in the middle of a trip to Greece should be fine but applying for a Greek visa and entering through Greece when the main goal of your trip is Paris (e.g. you spend two weeks in Paris with only a couple of days in Greece at either end of the trip to make the transfer) would be a blatant case of fraud and would expose you to all sorts of problems (including seeing your visa annulled and having to return to your country of residence immediately and probably having great difficulties in getting another Schengen visa in the future).
Even if you honestly changed your mind after getting the visa, if you spend more time in Paris that in Greece, it would look to an observer as if you lied to get in and chose to enter and leave through Greece to evade detection (in that scenario, if going to France for a long time was your plan all along, you were supposed to apply for a visa from France, not from Greece). Of course, since passports aren't always checked, it's possible to get away with this but planning to do it from the get go would nonetheless be an abuse of the system.
See also Should my first trip be to the country which issued my Schengen Visa?