That's smart of you to plan ahead! So a thick layer of snow will definitely block the sun and tank your production, but how much you lose really depends on where you live and how often you get lasting snow. Often, the panels will heat up and shed the snow on their own, especially in sunnier climates. But in the winter, you should expect lower overall production anyway due to shorter days and the sun being lower in the sky.
As for cleaning, the safest method is to let the panels shed the snow naturally.. if you have a ground-mounted system or can reach them safely, a soft-bristled broom or a purpose-built snow rake is an option. Just be careful not to scratch the glass or damage the panels. Never use a hard shovel or hot water! Remember, the main trade-off is the value of the energy you'd gain versus the effort and risk of cleaning them yourself.