Entirely pure.
‘Driven snow’ is snow that has blown into drifts and is untrodden and clean. Examples of the precise text ‘as pure as [the] driven snow’ aren’t found in print until around the start of the 19th century. Nevertheless, we have to thank Shakespeare for this popular simile. The complete phrase ‘as pure as the driven snow’ doesn’t appear in Shakespeare’s writing, but it almost does, and he used snow as a symbol for purity and whiteness in several plays. In The Winter’s Tale, 1611:

Autolycus: Lawn as white as driven snow.