While the sous vide method will give you a very easy to use environment for great results with minimal effort, it requires expensive equipment. I have invested 70 Euro worth of materials, many hours pushing bits in ugly C code, and one 220-volts accident, and mine is not ready yet :( Commercial ones are much more expensive, and frequently out of stock.
But people have been making fermented beverages in cold climates long before the first PID controller was built.
It is usually sufficient to heat the kvass in jars to a temperature which won't kill the bacteria in it, then pack it in some isolation. My grandma uses an old wool coat for her yogurt, but a container, possibly a beer cooler full of warm water, will give you a more stable environment. This will keep the liquid warm for many hours, until the fermentation is done.
Be aware that you will have to experiment a lot with the start temperature and the insulation to get the process to work well. If the temperature is lost too soon, the fermentation won't complete. If you start from too high a temperature, it will keep warm for longer, but you will get a rapid fermentation, which produces a very sharp tasting product. If you don't want the taste to be too strong, use thicker isolation and a lower starting temperature.