main.kv

#:kivy 1.0 

#:import SomeWidget main.SomeWidget

# rules for the widget
<SomeWidget>:
    BoxLayout:
        pos: root.pos
        size: root.size
        orientation: "vertical"
        Label:
            text: "hello"
        Button:
            text: "world"

# root
BoxLayout:
    id: layout
    Button:
        text: "add"
        on_press: layout.add_widget(SomeWidget())

main.py

#!/usr/bin/env python
from kivy.app import App 
from kivy.uix.widget import Widget
from kivy.factory import Factory


class SomeWidget(Widget):
    pass

Factory.register('SomeWidget', SomeWidget)

class Main(App):
    pass

Main().run()