Dear Trond,
it is actually possible to paddle from Pavia to Venice either in canoe, or in a raft. With a good kayak it takes from 7 to 10 days, depending on your paddling skills and physical shape.
With a raft, or a canadian canoe, for sure the time is more.
What is your experience with kayaks and canoes? Are you looking for someone to give you information about the trip, or actually someone who can carry you to your destination?
Hej då,
Stefano