The main difference between these two sour beers is in the vessel used to impart souring, as kettle souring involves fermenting beer with the aid of the probiotic bacteria (lactobacillus) in a kettle, while traditional sours use barrels of wood. The kettle souring process takes place in less than three days.
Traditional sours, on the other hand, involve leaving beers in barrels of wood with a mix of microbes to allow aging to occur. Where souring in (steel) kettle happens within a day or two, souring in (wood) barrels may take up to two years or more.