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An Award is given by some institution, body, Government etc. (A kind of prefix or degree is awarded to you e.g. Padmashree etc) Someone gets the Award for his contribution to wards society, sports, literature, bravery etc.
A Reward is something you will achieve through the results of your hard work ( e.g. one can feel rewarded for for his hard work by his achievements) it is more of a state of mind and
A Prize is something that you will receive in cash or in kind it may or may not indicate the efforts, contribution etc. Because you can win a Prize of Rs. 1,00,000/- by winning a lottery of Rs. 1,00,000/-.