Halvardsson was drafted in the 1999 NHL Entry Draft by the New York Islanders with their fourth choice, the 102nd overall selection. On May 15, 2006, Halvardsson signed a two-year contract, beginning season 2006–07, with the New York Islanders. He struggled in Islanders' affiliated club Bridgeport Sound Tigers in AHL with only eight games played. On December 1, 2006, he broke his contract and returned to Sweden, signing a one-year contract with his former Swedish club HV71. He later extended his contract with HV71 for another four years.
