Technically, diabetes mellitus is just hyperglycemia that spills into the urine if not treated - that was how it was originally diagnosed. Type I and II are actually completely different diseases that just both have the end result of hyperglycemia
Type I diabetes and LADA or type 1.5 have autoimmune destructon of islet islet b cells. Insulin sensitivity is usually normal.
Type II diabetes is a late consequence of the metabolic syndrome where the primary defect is in the liver - likely due to the neolithic agents fructose and n-6 PUFA injuring the liver and causing both liver and systemic inflammation. Impaired insulin sensitivity in the liver means the pancreas must secrete more insulin to communicate with the liver (control blood sugar, etc) - peripheral insulin resistance follows, likely as a defensive response to hyperinsulinemia and hyperglycemia (that the pancreas has trouble controlling despite increased insulin secretion) Ulimately, the hyperinsulinemia cannot keep up with the hyperglycemia, and when serum BG gets high enough, you get diabetes. The beta cells that make insulin fail, and in fact one of the things that damages them is hyperglycemia itself which is toxic to the beta cell. So you get B cell damage/death in type II eventually as well.
In my opinion, Type II diabetes theoretically be cured if it is caught and treated before there is too much beta cell destruction or burn-out. If one fixes the diet and allows enough time for the primary defect in liver sensitivity (and liver inflammation, NAFLD, etc) to heal, this metabolic defect should be reversible.
I am aware of at least one person who had type I diagnosed, and eating a paleo diet, the autoimmune reaction was arrested and they now no longer have diabetes. Usually type I is not diagnosed early enough for this to happen, but it could (and in the one case I am aware of did) reverse if caught right away and treated with a paleo diet.
So - Type I low insulin - high sBG - normal insulin sensitivity - can't be cured usually. VLC paleo is the treatment of choice as it allows more stable BG with less exogenous insulin
Type II - late effect of metabolic syndrome - can often be essentially cured. If caught early enough, you are cured, if later and the metabolism remains broken or pancreas is irreversibly burnt out, then VLC PaNu (Bernstein level of carbs) is the treatment of choice.