Bell also grabbed nine rebounds for the unbeaten Wildcats (7-0), who claimed the championship game in the Battle-4-Atlantis tournament.
Ennis' trey 1 minute into the overtime broke a 75-75 tie and put Villanova ahead to stay. He finished with eight points and four rebounds.
Josh Hart (14 points, five rebounds), JayVaughn Pinkston (14, seven), Ryan Arcidiacono (13 points) and Kris Jenkins (12) reached double figures for Villanova.
Roy Devyn Marble led the Hawkeyes (7-1) with 24 points.