"Boss" is a general word which means a person who's in charge of you at your work, s/he can also be the one who employs you. "Employer" addresses a person, company, or organization that employs people. "Owner" stands for a person or a group of people that hold the shares of a company so that they have the right to call the shots. Having said that, the owner can be the boss or the chairman of the company. Nevertheless, sometimes s/he would rather recruit another individual for that position. I hope this will meet your expectations

owner -own something employer -work for others and get paid boss- leader who leads from the front