Kaizen is a Japanese word for continuous improvement where Kai means change and Zen means for the better. For example a minor update to a piece of software
Kaikaku refers to radical change to the existing situation. For example upgrade of an application currently in use from a release level to a new release level
Kakushin refers to a complete departure from the current situation. It is about innovation, transformation, reform and renewal. For example replacing a complete application with a different application that supports the process in a completely different way. This kind of change will entail disappearance of many roles and functions within a business