"kk": "kk" is often used as a casual way to acknowledge receipt of a message or to indicate understanding. It's essentially a shortened form of "k" (which itself is a casual way of saying "okay"). The repetition of the letter 'k' adds emphasis to the acknowledgment.
"hmm" or "mm": Both "hmm" and "mm" are used to convey a thoughtful or contemplative response. They are often used when someone is considering a message, thinking about what to say next, or expressing a neutral or non-committal reaction. The choice between "hmm" and "mm" is usually a matter of personal preference or typing speed.
In general, these short responses are commonly used in informal digital communication to maintain a conversation's flow without providing lengthy or detailed responses. They can convey different tones or emotions based on the context and the sender's relationship with the recipient. For example, "kk" can sometimes be seen as dismissive if used in a formal or serious conversation, while "hmm" or "mm" might be perceived as a sign of attentiveness or consideration. However, it's essential to remember that the interpretation of these responses can vary between individuals and cultures.