PWM and CAN-bus in plain English
CAN-bus is the network that the control units in your car use to talk to each other. For the lighting, that means the on-board computer switches the bulbs and checks whether they work. If a bulb is dead, you get a warning on the dashboard.
PWM goes one step further. The on-board computer does not simply switch the power on, but switches it on and off very quickly. By changing the ratio between on and off, the car can dim the bulbs without you seeing any flicker. For a halogen bulb that is no problem: it simply glows a little less brightly. A xenon kit, however, has a ballast that expects a stable power supply. If that ballast receives a rapidly pulsing signal, the bulb can flicker, cut out or throw up an error message.
That is why it matters which kit you choose. A kit that is not made for PWM may seem to work on a modern car, yet still cause problems as soon as the on-board computer starts dimming or checking.