The klystron based high RF power system is a core element of the particle accelerator. The large-scale future accelerator projects such as FCC, CLIC and ILC will consume RF power at a level of hundred MW. Thus, the economical aspects of the machine operation, the environmental issues and the installation cost strongly depend on the efficiency performance of the RF power sources. The klystron technology has been developed for almost eight decades. For majority of the devices used in accelerator science it was established that the cost effective limit for the RF power production efficiency shall stay below 65%. In 2014, the dedicated study was initiated at CERN in attempt to improve the efficiency figure up to 80% and above through the thorough investigations of the physical process in the klystrons and understanding the limiting factors. As an outcome, a number of the new klystron bunching technologies have been developed and/or evaluated at CERN in collaboration with industry. These new technologies have been implemented into the klystron designs at different frequencies and power levels and predicted for some of them the efficiency reach as high as 80% (FCC CSM klystron for example). The general aspects of the new klystron technologies and their impact on the accelerators operation performance will be reviewed.