Cut the shrimp into small pieces—slightly smaller than 1/2 inch—and scoop into a serving bowl. In a blender, combine the lime juice, tomatillos, cilantro and 3/4 teaspoon salt. Blend until slushy looking, but not completely smooth. Stir into the shrimp and refrigerate 30 minutes. Just before serving, stir in the two chiles, chives and avocado. Taste and season with more salt if you think necessary.