What's the difference between "pain" and "painfulness"?
"pain" and "painfulness", both are nouns, so what's the differences between them? How to use them?
Also, another example is "joy" & "joyfulness".

Btw, which one is correct?
"eight points of happiness" or "eight points happiness"? Is there any better version for it?
And also, whether it is a single noun or a plural noun?
e.g "eight points happiness is gained/ are gained"??

Question

What's the difference between "pain" and "painfulness"?
"pain" and "painfulness", both are nouns, so what's the differences between them? How to use them?
Also, another example is "joy" & "joyfulness".

Btw, which one is correct? 
"eight points of happiness" or "eight points happiness"? Is there any better version for it?
And also, whether it is a single noun or a plural noun?
e.g "eight points happiness is gained/ are gained"??

Answer

Pain is more like the feeling itself whereas painfulness is more like describing the feeling or having the feeling. The formal definition for painfulness would be "the state or quality of being painful, of causing pain." For example you can say- I am in pain.-or-The painfulness of reading that book is hard to exaggerate. 
It would be "eight points of happiness are gained." This is because the subject of the sentence is "points" which is plural.
Hope this helps! -Amy