Xyzal is made with a derivative of the active ingredient in Zyrtec - it's basically a way to launch a new prescription drug that is nearly identical to one that is now over-the-counter (the same thing happened when Claritin became OTC and the manufacturer introduced Clarinex which is still a prescription).

To be really specific, Zyrtec's active ingredient is Cetirizine and Xyzal's is a derivative of Cetirizine called Levocetirizine Dihydrochloride.