'a felon' and 'a convict' are two common examples.

A 'hardened criminal' is a person who has committed crimes and is not sorry for having done so and plans on committing more crimes as long as they can. We usually associate the term more with violent or serious crimes. Bob and Jay rob a bank. They are caught. They are put in jail. While they are in jail, Jay realizes that he doesn't want to go to jail again, so when he gets out, he never commits another crime. Bob doesn't like being in jail either, but when he gets out, he robs a bank, but this time he shoots the witnesses on the way out and has a better getaway driver. They are both felons (bank robbery is a felony). They are both convicts (they have both been convicted and spent time in jail). Bob is a hardened criminal, because he's going to keep robbing banks. Jay is not.

Some other options for "hardened criminal" would be "a repeat offender" or "a habitual offender" This refers to someone who has often been sent to jail. Another term that I have often seen used in newspapers is, "a career criminal". This usually refers to someone who makes a profit from their crimes. "Jailbird" is someone, usually older, who has also been in jail for many years. "A juvenile offender" is a young person who has fallen short of the law a number of times. Hope this helps