Place the condensed milk, pumpkin and spices in a blender or food processor and blend until smooth.
Separately, whip the cream until it forms medium-firm peaks.
Carefully fold the pumpkin mixture into the cream so that it is combined but without loosing too much air.
Pour the mixture into a plastic container or loaf tin, cover and transfer to the freezer. Leave to freeze at least 4 hours for soft-serve or ideally overnight.
Notes
If possible, I'd recommend using homemade roasted pumpkin puree as it has a better depth of flavor. Roasted butternut or kabocha squash also work well.