Cavum, also frequently called "Hole Punch Cloud" or "Fallstreak Cloud", is caused when ice crystals are introduced into a thin cloud comprised of super-cooled water droplets, usually due to an aircraft passing through the cloud on takeoff or landing. The water droplets remain liquid well below freezing, since there is nothing present for them to freeze onto. With the sudden appearance of the ice crystals, the water droplets in the vicinity all rush in - creating a clear spot in the cloud - where they freeze and grow the crystals into snowflakes, which then fall out of the sky. The wispy clouds in the middle of the cavum, in the image below, are virga produced by that falling snow.
Cavum observed about 20km south of Linz, Austria. Source:wikicommons (H. Raab), 2008