I couldn't find the details for the train from Paris to Venice, but I have another suggestion for you, you may take the cheap domestic flight to fly from Paris to Venice, it saves much more time for you and if you book the tickets in advanced, you could always get the good deal, www.skyscanner.net
Once you choose the flight option then they will bring you through to the airline's own offical website and purchase the tickets. I will say it is very reliable, at least I bought all tickets many times and took those cheap airlines many times without any troubles.
But bear in mind that sometimes the airport located in somewhere far away from the city centre, you have to make sure you have another travel arrangement to get to the airport...