What is PackageKit?
From the official site:
PackageKit is a system designed to make installing and updating software on your computer easier. The primary design goal is to unify all the software graphical tools used in different distributions, and use some of the latest technology like PolicyKit.
The KDE program Discover and the GNOME program gnome-software use PackageKit to install packages on your system. Since distributions use different package managers (apt, zypper, dnf, etc) PackageKit makes it easier for installers to achieve this.
However some users prefer to always use Zypper on the terminal and have actually no need for PackageKit. It can even annoy them since sometimes PackageKit can perform an action in the background and thus they can't use zypper and get a notice:
PackageKit is still running (probably busy)
How to disable PackageKit
To stop and disable the systemd service:
sudo systemctl disable --now packagekit
It might be that some program (like gnome-software) is trying to restart it. For such cases we can mask it:
sudo systemctl mask packagekit
To completely remove it:
sudo zypper remove PackageKit