How do you write the diary that belongs to kim in possessive form?

The phrase "Kim's diary" is already in the possessive form.

Here are a few ways to write it, depending on the context:

Formal:

* Kim's diary

* The diary belonging to Kim

Informal:

* Kim's journal

* That diary of Kim's

It's important to choose the phrasing that best suits the tone and context of your writing.