Siva is dahi (yogurt) and Krsna is milk. Dahi, the preparation of dahi that you take milk, pure milk, and mix it with little sour thing, then it becomes dahi. Similarly, Krsna is pure spirit, and when this pure spirit is mixed up with maya, that is Siva.

From Srila Prabhupada’s lecture on Srimad-Bhagavatam 5.5.2 — Hyderabad, April 13, 1975