Glycerol usually comes in a liquid form. So 50% v/v would be 50 mL of glycerol in a 100 mL solution. A 50% w/v would be 50 g per 100 mL of solution. So basicly you measure your glycerol by weight (50 g) or by volume (50 mL), depending on your solution, then raise the volume to 100 mL with ddH20.
So if you known the density, all you have to do is calculate the amount of g in a 50% v/v solution and compare it to your 50% w/v solution. You can also calculate the density of both solution and compare it. Basicly one solution has more glycerol than the other one.