There is no difference between 'healthy' and 'healthful', however I will say that 'healthful' is a very obscure word. I have never heard it used before, and the only reason I even believe that it is a real word now is because it is in dictionaries. For natural use, stick to 'healthy'.

Healthful describes something that gives good health, whereas healthy describes the good state of health of someone or something. For example: "I have chosen the most healthful foods to cook with." "Fish is a healthful alternative to meat" "I am healthy" "The tree is healthy" "Running is a healthy habit"