Hi, Mukesh.
There are discount flights from several airlines including Ryanair. There's a flight that goes out of Venice to Rome and can be really inexpensive. READ UP first though. The discount airlines can be really extreme on baggage conditions - especially Ryanair. There's a lot of good information on here about. They fly Rome - to outside apris too (it's a bit of a bus trip I understand).
IF you follow their rules, (and ask questions in advance), you can get really cheap flight(s), and my 3 experiences with them have been good.
Train and Car - Train can be pricey - but there is a nice overnight train from Venice to Rome - save a day, and save a night's hotel fair. (It's around $75USD per person for a 4 sleeper car).
Car - You can't drive in Venice, and there are several places to drop off outside of Rome (which you will want to do). So it/s all strait freeway. One way drop off fees are high - but compared to 7 train tickets, not really.
Either train or car, you can stop in Florence. It's along the way, just around the halfway mark (close enough!). It's not a bad day trip from Rome either (2 hrs).