Would have done: Here the implication is that the subject of the sentence(doer) was not in a position to do action due to things beyond their control.

• I would have helped him, but I was not having surplus cash with me.
  (Here I was not able to help, I may have had the desire to help him).
• She would have called me, but she didn’t have her mobile phone with her.
  (She was not able to phone me as she didn’t have the mobile phone with her)
• The police would have saved the victims, but they didn’t have a boat.
  (Actually, the police were unable to save them due to lack of a boat)

Could have done: Here the subject of the sentence(doer) was very much in a position to do things, but they didn’t want to do or refused to do.

• I could have helped him, but he was a dishonest man.
  (Here I was able to help, but I chose not to).
• She could have called me, but she wasn’t on good terms with me.
  (Here she was able to call me, but she didn’t want to).
• The police could have saved the victims, but the victims refused to move from their homes.
  (Here the police were able to save the victims, but the victims refused to move).