Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
Palace are thought to be open to offers for Garvan with the playmaker entering the final year of his contract at Selhurst Park.
Garvan played a key role in helping Palace win promotion to the Premier League in the 2012/13 season, but was then controversially left out of their 25-man squad last season.
The former Republic of Ireland international enjoyed a two-month spell on loan at Millwall at the back end of last season and helped the Lions secure their survival in the Championship.
Millwall are thought to be one of the clubs chasing Garvan, but they could face a battle for his signature with several other clubs keeping tabs on the Irishman's situation at Selhurst Park.