Elven is a unique and obscure language. It is known by the half-elf banshee Ielephia and some inhabitants living on Mount Hyjal. This is the fourth named elven dialect after Darnassian, Thalassian, and Nazja. There are four known elven dialects.
This language is not to be confused with Thalassian which is sometimes referred to as "High Elven" by individuals who are unfamiliar with its proper name.
"Elven" is a language derived from Dungeons and Dragons, which can be found in the DnD rule books. In DnD it is the language used by various types of elves, and uses the Elven alphabet. The language is based on the languages of the elves in Lord of the Rings. In the Warcraft RPG it is listed as a separate language from Darnassian, Nazja and Thalassian.
Additionally, Cycle of Hatred uses the term "elven" to describe four total distinct dialects used by elves, and races descended from elves. However, it does not list the four dialects by name. It can be assumed that they would include Thalassian, Nazja, Darnassian, and a final mystery language. As "Elven" is the only other elven dialect ever mention it is likely the fourth language...however, it might be some as of yet unknown language.
It may be the "strange dialect of Darnassian" found on ancient ruins. Its unknown if that mysterious language was derived from or predates the current form of Darnassian. It is perhaps the language of the ancient dark elves the only other elven race mentioned in lore.
It is possible that elven is the language from which all current elf languages derive, spoken by the Kaldorei before the Great Sundering. Then the other four dialects: Nazja, Thalassian, Darnassian, and the final, mystery language evolved over the 10,000 year period after the War of the Ancients.