Next vacations(febuary 2013) I will stay 3 days on Paris with other 3 people. One day on Louvre, next day on city and another one... is still a doubt... We are wondering about going to London from Paris on morning, spend the day on London city and returning back to Paris at night, it is possible and feasible?
It's certainly possible to do a day-trip from Paris to London. Eurostar takes a little over 2 hours each way from Paris (Gare du Nord) to London (St Pancras) and back. In both directions you clear immigration (both exit and entry!) before boarding the train, which adds a little to the total time.
However, if you've never been to Paris before, I'm really not sure that I'd suggest doing this. Paris is an amazing city, and there's more than enough to do in 3 days (or even 3 weeks!) that I wouldn't generally recommend spending 3 hours (and a non-trivial amount of money) getting to London, just to turn around and go back!
Note that this isn't saying anything against London - if you were in London and considering a day trip to Paris I'd probably be saying the same thing :)
Well it's POSSIBLE. The train ride is 2.5 hours, roughly, plus time in customs/waiting. So yeah, you could do it.
However, should you? No. If you only have three days, don't spend what's realistically 8 hours once you include waiting and travel time, unless you're a real train fan and want to try out the Eurostar.
IF however, you do, then you'll arrive at King's Cross / St Pancras in London. You could feasibly get to see Tower Bridge, Big Ben, ride the London Eye (if you get a short queue), and explore one of the parks, and walk past Buckingham Palace if you really wanted. Which sounds like a lot, but it's all quite close together.
I had a short weekend in Paris a few years back for a friend's birthday, before continuing on to Slovakia. I still want to go back - Paris seriously has SO much to do and see. Moulin Rouge, walking the river, checking out some markets, Notre Dame, catacombs (those are really worth seeing!), and more. I say use those valuable hours to see Paris in more detail, rather than potentially rushing two cities.
Since the trip on the Eurostar to London from Paris is 2:22 and back is slightly longer at 2:27 it is certainly feasible to make a day trip to Paris from London. The earliest train on August 1, for example is at 07:13, arriving at 08:40; and the last return trip from London to Paris leaves at 20:55, getting to Paris at 00:17.
Tickets on the Eurostar aren't exactly cheap—starting at around £58.00 each way, so it might be better to spend the day in Paris, perhaps Versailles, which is accessible by RER and the Metro