Paris Hilton and her boyfriend, Greek shipping heir Stavros Niarchos, have split, Hilton's publicist Lori Glass, tells PEOPLE.

"I confirm that she's split with him. They are no longer a couple. I don't know the exact day they split."

Hilton, 25, and Niarchos, 21, began dating last year.

As for rumors of a possible relationship with newly drafted Arizona Cardinal Matt Leinart, "She knows Matt, she likes Matt. They are friends," Elliot Mintz, another publicist for Hilton, tells PEOPLE. "I don't want to go any further than that. They have known each other a while."

Last month Hilton threw Niarchos a surprise 21st birthday bash at her Hollywood Hills pad. Some 100 guests turned out for the party, during which "Stavros did back flips into the pool and jumped from one end to the other," said a guest. "Everyone gasped and begged him not to continue because his stunts looked dangerous."

The happy couple were inseparable that night, holding hands and kissing.

Niarchos was previously involved with Mary-Kate Olsen, whom he left for Hilton.

For her part, Hilton was previously engaged to Paris Latsis, also a Greek shipping heir.