Wash and quarter the red potatoes. Smack the garlic cloves with the side of a kitchen knife to open and place in a large stockpot or Dutch oven with the potatoes and cover with water. Add kosher salt and bring to a boil over high heat, cook until fork tender. Drain and set aside.
Return pan to medium-high heat, cook and render the bacon until crisp. Set aside to cool slightly and chop. Keep bacon fat in pan, add the cabbage and kale and cook until wilted, about 5 minutes.
Add the potatoes back to the pan. Add the milk, cream, and black pepper, and bring to a boil. Using a potato masher, mash and stir potatoes until smooth and thoroughly incorporated with cabbage and kale. Add the bacon crumbles and season with kosher salt if needed. Transfer to a bowl, top with chopped chives and serve hot with butter on top.