Man of the match Michael Kightly had the first chance after two minutes, an 18-yard left-foot shot which saved and then in the sixth minute he had a six-yard shot which was pushed away by Preston keeper Andy Lonergan.
Preston's best player Neil Mellor had a cross saved by the Wolves keeper after quarter-of-an-hour.
Iwelumo hit a 25-yard effort that was saved and then Sylvan Ebanks-Blake had an 18-yard shot that was kept out by Preston keeper Andy Lonergan.
Wolves opened the scoring after 36 minutes when a David Jones corner was headed against the bar by Neill Collins and Jones was then allowed to find Iwelumo who fired home.
Ebanks-Blake and Kevin Foley had efforts saved by the Preston keeper and Kightly had a corner that was punched away.
After the interval, Jon Parkin had a 12-yard header saved and Michael Hart had a cross caught by Wolves keeper Wayne Hennessey.
Wolves made it 2-0 when Kightly crossed for Iwelumo to hit a right-foot shot into the bottom right-hand corner of the net from six yards.
Lonergan tipped an Iwelumo header around the post and also saved Karl Henry's 12-yard shot with his feet.
Wolves won a penalty when Jones fouled Kightly and Iwelumo completed his hat-trick with a right-foot penalty.
Iwelumo was sent off for violent conduct after a foul on Sean St Ledger and, in stoppage time, Hennessey brought down Mellor in the penalty area and was sent off.
Mellor hit a right-foot penalty into the bottom left-hand corner of the net but it was no more than a consolation goal for Alan Irvine's side.