It is a mistake to think that Ivo is a diminutive for Ivan (in Bulgaria, Serbia). Ivo & Ivan are quite different names. The name which is the most close to Ivo, is Ivailo (Ivaylo). Very often, they use "Ivo" as a diminutive for "Ivailo". In the same time, they never refer to anyone Ivo as "Ivailo". It may seems a bit complicated for non-Bulgarian people but that is that. :)
The old Czech Christian meaning of the name "Ivo" is something like "God is Merciful" but mostly it is meant to be a yew or a long-bow made out of yew. The origin is Old-German and Czech/Bohemian. Saint Ivo lived in France in the 14th century. He was a lawyer helping & providing legal counselling to the poor. You can see his statue in Prague. It's the 1st statue on the left side while you are heading the Prague Castle from Old Town. BTW my name's Ivo & I'm from Czech Republic.