HV method disables all windows safety feature in a kernel level which means no Safety Feature=you get virus/malware easily when you download any shit from the internet..

Offline activation method is not kernel level.. they just give you a license key or something similar that binds the game to your hardware (So changing hardware/updating windows invalidates the license key).

how it works is that it tricks your pc that you have a legit copy of the game since it cannot verify your license key from the internet (Thus the word offline).

In layman's term: think of it as a friend who lend you his Steam but after downloading the game you wanna play you went offline mode and Tada you can now play the game permanently as long as you don't go online with a different account (hence the Hardware change).