both are synonyms of each other, although keen can have other meanings: eager (adj.) - motivated and enthusiastic about something (i.e He is eager to begin his English course.) keen (adj.) - eager (i.e He is keen to begin his English course.) to be sharp, intellectually quick (i.e He has a keen wit.) intense or strong (i.e He has a keen desire to pass his English exam.)

On this site it is describe what is the difference of this two words :) http://www.usingenglish.com/forum/ask-teacher/82749-difference-between-keen-eager.html