KNX is a well adopted standard for domotics using a specific communication wire (although it can also use RF and Powerline). KNX is complex, not cheap and is mostly used in business or hotel space automation.
ZigBee and Z-Wave are two RF (radio frequency) domotics technologies. Z-Wave is more adopted worldwide (thousands of equipment to choose from ), has an average price (around 50 USD for each sensor or actuator) and could be simpler to configure. Usually the hassle in Z-Wave is with “pairing” equipment and the main controller. ZigBee has some penetration in the US, is now becoming interesting due to moves from vendors like , but still doesn’t have as many equipment vendors as Z-Wave and as the standard is only for lower level communication many ZigBee equipments are not compatible among each other. Z-Wave devices are compatible given they belong to the same region (e.g. US, EU, etc.).
Both Z-Wave and Zigbee are technologically interesting as they allow for battery powered devices to be deployed wirelessly and run autonomously for years (months only in many cases) without battery replacement. Also, you can grow your domotics network as needed and newer equipment is available. KNX doesn’t support this so well, it’s more tailored for new builds.
There are also interesting but proprietary alternatives to Z-Wave (HomeEasy –, HomeMatic and MAX! , D-IO: ) that are cheaper and new projects seem promising for the near future Thread – .
For now, in a new build, I’m considering using Z-Wave devices as there is more choice in the market, it’s not a single vendor proprietary solution (as far as equipment goes, RF modules are built by two vendors only) and even though proprietary solutions can be cheaper they don’t offer routing between equipment that Z-Wave and ZigBee do, so you might have hard to solve networking problems.
Many automation integrators / platforms support all 3 (KNX, Z-Wave, Zigbee) and more:
Some general references: