I can't running "hello world" with Kivy and virtualenv - python-3.x

Question

I'm installed kivy with python3 and virtualenv following the official documentation
The 'showcase' example run perfectly but if I create a simple app and running it I don't get any error and the display not showing anything. I think the error is related to the use of virtualenv, but I don't understand why the showcase example works.
This is the kivy log:
[INFO ] [Logger ] Record log in /home/pi/.kivy/logs/kivy_23-02-17_55.txt
[INFO ] [Kivy ] v2.1.0
[INFO ] [Kivy ] Installed at "/home/pi/kivy_venv/lib/python3.7/site-packages/kivy/__init__.py"
[INFO ] [Python ] v3.7.3 (default, Oct 31 2022, 14:04:00)
[GCC 8.3.0]
[INFO ] [Python ] Interpreter at "/home/pi/kivy_venv/bin/python"
[INFO ] [Logger ] Purge log fired. Processing...
[INFO ] [Logger ] Purge finished!
[INFO ] [Factory ] 189 symbols loaded
[INFO ] [Image ] Providers: img_tex, img_dds, img_sdl2, img_pil (img_ffpyplayer ignored)
[INFO ] [Window ] Provider: sdl2
[INFO ] [GL ] Using the "OpenGL" graphics system
[INFO ] [GL ] Backend used <sdl2>
[INFO ] [GL ] OpenGL version <b'OpenGL ES 3.1 Mesa 19.3.2'>
[INFO ] [GL ] OpenGL vendor <b'VMware, Inc.'>
[INFO ] [GL ] OpenGL renderer <b'llvmpipe (LLVM 9.0.1, 128 bits)'>
[INFO ] [GL ] OpenGL parsed version: 3, 1
[INFO ] [GL ] Shading version <b'OpenGL ES GLSL ES 3.10'>
[INFO ] [GL ] Texture max size <8192>
[INFO ] [GL ] Texture max units <32>
[INFO ] [Window ] auto add sdl2 input provider
[INFO ] [Window ] virtual keyboard not allowed, single mode, not docked
[INFO ] [Text ] Provider: sdl2(['text_pango'] ignored)
[INFO ] [ProbeSysfs ] device match: /dev/input/event0
[INFO ] [MTD ] Read event from </dev/input/event0>
[INFO ] [MTD ] Set custom rotation to 180
[INFO ] [MTD ] Set custom invert_y to 0
[INFO ] [ProbeSysfs ] device match: /dev/input/event0
[INFO ] [ProbeSysfs ] Unable to find provider hdinput
[INFO ] [ProbeSysfs ] fallback on hidinput
[INFO ] [HIDInput ] Read event from </dev/input/event0>
[INFO ] [HIDInput ] Set custom rotation to 180
[INFO ] [HIDInput ] Set custom invert_y to 0
[INFO ] [ProbeSysfs ] device match: /dev/input/event0
[INFO ] [HIDInput ] Read event from </dev/input/event0>
[INFO ] [HIDInput ] Set custom rotation to 180
[INFO ] [Base ] Start application main loop
[INFO ] [HIDMotionEvent] using <raspberrypi-ts>
[INFO ] [HIDMotionEvent] using <raspberrypi-ts>
[INFO ] [MTD ] </dev/input/event0> range position X is 0 - 799
[INFO ] [GL ] NPOT texture support is available
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range ABS X position is 0 - 799
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range ABS X position is 0 - 799
[INFO ] [MTD ] </dev/input/event0> range position Y is 0 - 479
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range ABS Y position is 0 - 479
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range ABS Y position is 0 - 479
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range position X is 0 - 799
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range position X is 0 - 799
[INFO ] [MTD ] </dev/input/event0> range touch major is 0 - 0
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range position Y is 0 - 479
[INFO ] [HIDMotionEvent] <raspberrypi-ts> range position Y is 0 - 479
[INFO ] [MTD ] </dev/input/event0> range touch minor is 0 - 0
[INFO ] [MTD ] </dev/input/event0> range pressure is 0 - 255
[INFO ] [MTD ] </dev/input/event0> axes invertion: X is 0, Y is 0
[INFO ] [MTD ] </dev/input/event0> rotation set to 180
And this are the code files:
main.py
from kivy.app import App
class HelloApp(App):
pass
if __name__ == '__main__':
HelloApp().run()
hello.kv
Label:
text: "Hello World"