There is no official / consensus answer to this question, among Egyptologists. The most likely candidate for Tut'ankhamun's original tomb is KV23 (aka WV23), which became the tomb of King Aye. There are a couple reasons why historians suspect this might be the tomb initiated by Tut'ankhamun.

1. The KV23 tomb is large. Much larger than we would expect for Aye, who only ruled 4 years or so. If the tomb was underway when Tut'ankhamun died, then Aye completed it, that would make sense regarding the size, etc.
2. Location. The KV23 tomb is in the Western Valley, near the tomb of Amunhotep III. King Tut'ankhamun connected himself with Amunhotep a lot in his reign. He referenced that king repeatedly, as part of his official policy. So, putting his tomb near Amunhotep III would make sense as a connection between the two.
3. Design. The KV23 tomb is a direct continuation of the architectural design that appeared with Akhenaten. From the reign of Akhenaten onwards, royal tombs followed a straight axis, directly into the cliffs/earth. Aye's tomb is similar to the one used for Tut'ankhamun in a couple respects, but with a much longer entrance / corridor sequence. (Also, the art is a direct continuation of the style used in Tut'ankhamun's tomb. But that is to be expected. Probably the same artist did both, since they were painted just 5 years apart).

None of these are conclusive. Merely circumstantial. So, without inscriptions (like artefacts or ostraca, recovered from the area near KV23), it is merely a hypothesis. A decent hypothesis, but still not reliable.