I found some in the European section of a store. From the ingredient list it is rye bread soaked in seltzer water. Reading up a bit on it there should be a small amount of alcohol. Taste is described as pickley and gamey. In a good sense.
Yeah, I used to live in Russia, it’s a common summer drink. It’s kind of like kombucha.
I live in a neighborhood flled with exSoviets. I’ve brought KVAS to a few Dopfests. I’ve never felt the urge to try it myself. A friend told me it is similar to Malta Goya, a beverage aimed at the Hispanic market. I’ve never tried that either.
I too used to live full-time in Russia, where kvas is available year-round and sold on the street in the summer. It’s basically near-beer made from rye bread. It’s naturally carbonated and mildly alcoholic—not really enough to get a buzz on, though. The flavor is quite nice, kind of sour and sweet at the same time.
It’s also very refreshing, especially when chilled. Locally produced (Canadian) kvas is sold at Highland Farms supermarket here in Scarborough. Whenever I go shopping there, I treat myself to a small bottle and relax in their little coffee corner before heading home.
Give it a try if you haven’t already. 
PS: It is similar to mildly carbonated malt beverages, which are also very good.
I tried some in Lithuania. It was … weird. Sort of like bread-flavored soda. I think it’s an acquired taste.
I may as well try some next time I go out to the store. I’ve tried weirder. OTTOMH I bought a can of “white fungus drink” at a Chinese market. It delivered what it promised.
Recently, I thought “Tvorog” meant cottage cheese. It’s similar. But tvorog is much more sour, bitter and salty.
As I have retold previously “curd cheese bar” turned out to be chocolate coated cheesecake bites.
Kvas is also used to make okroshka, a cold soup that’s very refreshing in the summer. I first had it in Riga, way back in July 1975.
I don’t know what kind of tvorog you bought, but I’ve never had any that’s sour, bitter, or salty. I’ve actually used the fresh stuff as a substitute for ricotta in making lasagna. It differs from cottage cheese mainly in being all curds and no whey.
Syrki snacks can be tangy; they’re definitely not as sweet as cheesecake. I like them, and my daughter (born in Moscow) grew up with them.
I’ve seen kvas at the Yonge/Steeles No Frills (seasonally, I guess). I have considered buying some, but not seriously.
I’ve brewed kvass, but have never had anything authentic.
Basically, one uses leftover rye bread to carbonate a “sour” drink that is less than 1% alcohol. You can read up on the Mad Fermentationists take here: Kvass - Liquid Sourdough Beer | The Mad Fermentationist - Homebrewing Blog
If you want to see the varieties of Kvas head to the Yummy Market on Dufferin south of Finch (where the Canadian Tire gas bar is located). Also for anything else Eastern European including fresh bread.
I have some friends who made a batch a couple years ago. It tasted just like drinking a loaf of rye bread. The kind with the caraway seeds in it.