Paris Hilton has apologized to former pal Kim Kardashian after poking fun at the reality TV star's famous bottom live on air.

In a recent radio interview, Hilton insisted she'd never swap butts with the voluptuous former Playboy pin-up, likening Kardashian's best asset to "cottage cheese inside a big trash bag," and adding, "It's gross".

When Hilton's namesake blogger Perez Hilton got hold of the transcript from the radio interview and posted it on his website on Monday, the socialite was horrified and has since apologised to Kardashian.

She tells In Touch magazine, "I was just joking around and I made a stupid joke. I felt really bad afterward, so I contacted Kim and apologised. It was a silly thing to say. Kim's hot!"

Kardashian has confirmed Hilton called to apologize, adding, "Paris and I have been friends since we were kids, and I'm glad she made the effort to say she's sorry."