There are so many acronyms, slangs online, getting to know all of them be extremely difficult, but knowing the popular ones might savage the situation. One of those popular acronyms is TYVM which means Thank You Very Much.
There is no way something good has never been done to you before and how do you appreciate the gesture?, do you still go through the rigor of typing 'thank you very much'?. Instead of typing the full form you can simply type TYVM and it will deliver the same meaning. This type of slangs is commonly seen online especially when people are chatting with their friends.
They use it to show that the kind gesture extended to them is deeply appreciated. But to me, that is too casual, if indeed you appreciate what someone did for you, it shouldn't cost you anything to write the full form of TYVM. Now that you've known the full form of TYVM, you can use it as you like.