Evidence indicates that in 1950, kalimba and karimba were used more or less interchangeably to describe any traditional African thumb piano. Seventy years ago, mbira dzavadzimu meant exactly what it does now - a particular traditional Shona thumb piano - and the mbira nyunga nyunga probably was not even a thing yet. Seventy years ago was just before Hugh Tracey began to build and sell a new instrument which he called a kalimba, which combined features of many traditional instruments and had a western tuning.
Thumb pianos evolved over time and were affected by numerous influences, ancient and modern, African, European, etc. They have a very rich, varied genealogy. Today, kalimba usually refers to non-traditional thumb pianos. Karimba refers to a particular family of traditional African instruments also known as the African-tuned karimba. Mbira nyunga nyunga, which translates to the "sparkly sparkly mbira" is essentially the same instrument as the karimba, though if it is rustic-looking, it is more likely to be called mbira nyunga nyunga, and a workshop-made instrument might be more likely to be called karimba.
But it isn't exactly that simple, and here I will outline the details of this fundamental discussion.