Ritul, Not really much difference. Just 2 small ones: Both imply "possible" or "hypothetical" actions. 1. "Could have been done..." can be used to emphasize "possible completion in the past" of the hypothetical action. Example: If my car was broken, and my dad has fixed it, but I also know how to fix the car I might say to him: "Dad, I could have done that. You don't need to fix it for me." If my sister wants me to take her to a movie, but she has to wait for me to finish my homework she might say to me: "You know, that homework could have been done yesterday." (emphasizing completion in the past). 2. "Could have been done" is in "passive voice." "Could have done" is in "active voice." In active voice the subject is said or emphasized. In passive voice, the subject is not stated or emphasized. Example: Active voice - "The doctor could have done the surgery, but he didn't arrive on time." (Who? -- the doctor!) Passive voice - "A surgery could have been done to save the women, but she died." ("something could have been done"--- by WHO? We don't know).

They both are referring to the same sort of situation, it's more a matter of what they apply to. COULD HAVE DONE: the subject is the person doing the action. I would have been difficult, but John could have done it. COULD HAVE BEEN DONE: the subject is the action itself. The construction of the pyramids a Giza could have been done only by a highly organized society.

"Could have done" means that the person had the opportunity to do something earlier. Ex. If I did not wash my car yesterday morning, I could have done my shopping before noon. "Could have been done" means that the person had the opportunity to be finished or completed Ex. If I read the book last night, I could have been done the assignment before history class.