K-Cav's real arch-nemesis isn't Audrina or Stephanie...it's Lo!

Although Kristin Cavallari most often battles with Audrina Patridge on The Hills, off screen it's Lauren "Lo" Bosworth she can't stand.

"Kristin hates her," a source tells Us Weekly of Lauren Conrad's best friend (and current roommate).

"Lo has said some really nasty things about Kristin, and it got back to her. She's OK with most of the other girls, but she legitimately hates Lo."

Just as surprising: Cavallari and Patridge now get along famously. It turns out the animosity between the pair -- which began when Cavallari dated Patridge's ex Justin "Bobby" Brescia -- may now be just for the cameras.

A source says the two women are decent to each other and, gulp, act like friends in real life! They even worked out together May 3.

"Audrina and Kristin really get along." In fact, the source says Cavallari admires Patridge: "Kristin kisses up to Audrina! They don't have drama anymore."

To find out more Hills scoop -- including how the female cast brainstormed about the finale with show creator Adam DiVello, what's next for Heidi and Spencer Pratt, and more -- pick up the new Us Weekly, on newsstands now.