Slice 2 kiwis crosswise into 1/2-inch-thick rounds, then cut each round into quarters; reserve for garnish.
Cut remaining 6 kiwis into large pieces. Using an electric juicer, juice kiwis, grapes, and apples, and transfer to a medium bowl. Stir in lemon juice. Transfer to a pitcher filled with ice.
Divide juice among 4 glasses. Garnish with kiwi, and serve immediately.