reference: Very intuitive example shown in this blog

https://wiseodd.github.io/techblog/2016/12/21/forward-reverse-kl/

Let our true distribution defined as $P(X)$, and the approximate distribution as $Q(X)$.
Forward KL: $\sum_{x \in X} P(x) \log(\frac{P(x)}{Q(x)})$
Discussion in different cases:

1. If P(x)=0, then log term can be igored so that the Q(x) can be any shape when P(x)=0. Q(x) is able to assign any probabilities when P(x)=0)
2. If P(x)>0, then the log term have effect during optimization so that Q(x) assign probabilities as close as possible to P(X) when P(x)>0.
3. The following graph is a specific optimal optimization for Forward KL.
   

Reverse KL: $\sum_{x \in X} Q(x) \log(\frac{Q(x)}{P(x)})$

1. If Q(x)=0, log term can be ignored so that Q(x) able to assign 0 probabilities to P(x)>=0.
2. If Q(x) > 0, log term is taken into account in optimization step so that Q(x) assign probabilites as close as possible to P(X) when P(x)>0
3. The following graph is a specific optimal optimization for Reverse KL.