The best and most recognized spot in Sicily is called Stagnone (Marsala) close to Trapani - From Palermo I think it is not far.
If you go from Palermo to Marsala by car I am sure will be easy to find the spot and a school to rent kites and boards. Average wind is 15-20 knots and it is full of italian pro kiters.
The other spots are on the south east cost, less crowded, no hassles, a lot of winds, but also would be difficult to find rentals.
150 Km from Etna and from Agrigento
Actually in any place between Pozzallo (Ragusa) and Donnalucata (Ragusa) you can find kiters on the beach every month.
More info on
http://www.kitesicilia.it (partially in english) or
info@kitesicilia.it
have fun[/url]