A tax haven is a place where companies and/or individuals pay very low taxes (or no taxes at all). In most languages, such a place is literally called a “tax paradise”, e.g.
Paradis fiscal (French)
Paraíso fiscal (Spanish)
Haven, pronounced /ˈheɪvn/ (hey-vn), means “a safe place; a shelter”. It is a relatively uncommon word (about 10x less common than “heaven”), and the only difference between “haven” and “heaven” is the letter “e”.
Since paradise and heaven are closely related concepts, non-native speakers of English who are not familiar with the word “haven” often simply don’t notice the spelling difference and automatically assume that a “tax haven” is actually a “tax heaven”.
However, rest assured that the only standard form in English is tax haven. “Tax heaven” is an incorrect back-formation from the equivalent expressions in other languages and is generally frowned upon in all kinds of academic writing.