What is the difference between future perfect continuous and future perfect tense?

The word been is in perfect continuous and not future perfect tense. This is the first step in deciding the tense present.

Future perfect have a structure as will + have + verb ending in en Future perfect continuous has a structure as will +have + been+verb ending in ing

eg. Harry writes a letter.

Future perfect: Harry will have written a letter.

Future perfect continuous: Harry will have been writing a letter.