Despite being listed as one volcano, they are actually separate from each other. Avachinsky and Koryaksky are both part of the Avachinskaya volcanic group which is a group of stratovolcanoes which are orientated in a NW-SE direction. The other prominent volcano in this range is Kozhel. These volcanoes are the result of the Pacific plate subducting beneath the North American plate at a rate of ~ 78 mm a year. Due to the climate in the area and the amount of ice and snow on the volcanoes the main hazard posed from Avachinsky and Koryaksy is by lahars with Pyroclastic Density Currents a close second.
The town present in the foreground of the photos below is Petropavlovsk.
Avachinsky is a stratovolcano which stands at ~2741 m high. The last eruption of this volcano was a tiny eruption in 2001 with another small eruption in 1991. An event in 1945 was much larger and was rated at VEI 4. The eruptions produced by Avachinsky are generally smaller in scale at ~VEI 2. Avachinsky also houses an active crater with regular degassing and fumarolic episodes.