How would you travel from Paris to Venice?

EasyJet, Air Berlin, Air France, and Alitalia, all have direct flights.

http://www.kayak.com

Discount airline easyjet flies from CDG and Orly to Marco Polo airport....the earlier you book, the cheaper the fare. Right now, they're offering fares as low as about 27 Euro pp.

Also Ryanair, but have to be careful of luggage, also probably with other ones too

And Ryanair does not fly to the main airports near either Paris nor Venice. Both are a hike into town. I'd stick with the other airlines for this route.