The difference between Marasmus and Kwashiorkor is that 

Marasmus is caused by deficiency of Vitamin B while Kwashiorkor is caused by the deficiency of Vitamin D.

Marasmus is caused by a calorie deficiency while Kwashiorkor is caused by protein deficiency.

Marasmus is a simultaneous deficiency of proteins and calories while Kwashiorkor is due to just protein deficiency unaccompanied by calorie deficiency.

Marasmus is a deficiency of just proteins while Kwashiorkor is due to a deficiency of both proteins and calories.