A number of Europe's top clubs including Madrid, Manchester United, Chelsea, Liverpool and Bayern Munich have all been linked with the attacking midfielder, who scored 12 goals in 30 Bundesliga appearances in the 2019-20 campaign.
The 21-year-old is due to face Bayern in Saturday's DFB-Pokal final clash and while Calmund thinks that the Bavarians are interested in signing Havertz, he believes he would prefer a switch to Spain.
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He told Goal: "From his end it has been suggested that Spain is his preferred destination. Barcelona are currently unable to fund a transfer of this magnitude.
"I would not recommend him to move to Barca either, he would always be compared with Lionel Messi and the media and the fans only like to remember the outstanding performances of Messi.
"Real Madrid is a more logical destination. I think Real are very interested in Kai, but they are also struggling to sign him this summer. In my view, that's an advantage. If you switch to such a world-renowned club at a young age, you must get a regular place in the team.
"The chances of that happening next year are very good at Madrid. Luka Modric, the one who plays in Kai's position, will be 36 years old next September. That would be the right time for a change.
"To my knowledge, Real can also imagine such a scenario. And if it is next year, that would mean that Kai will play another season with Leverkusen.
"Of course, FC Bayern are also interested in him. But I cannot imagine that they will be able to spend around €100 million (£90m/$112m) after signing Leroy Sane. I would like to see Kai continue to play in Germany, and if that were to happen then only FC Bayern could be be considered. Maybe a deal for next year is possible."
Calmund also discussed Havertz's valuation, with it his opinion that Leverkusen will not look to sell for less than €100m, while he thinks he is more talented than Michael Ballack was at the same age.
He added: "Havertz scores goals with his left foot, right foot and his head. That is an exceptional quality to have. He is also versatile, fit, technically strong and fast. He is an exceptional talent.
"The two Bayer 04 managing directors Fernando Carro and Rudi Voller, like the supervisory board chairman Werner Wenning, have committed themselves to €100m as the basis for negotiations. The three Bayer bosses will not move significantly away from this amount.
"At Leverkusen (during Calmund's time at the club) we had three absolutely exceptional midfielders in Michael Ballack, Emerson and Bernd Schuster, whose careers I have followed intensively from the start.
"Ballack was the most dangerous, Emerson shone with his versatility and Schuster was great from a strategic point of view. If you compare these guys to Havertz when they were 21 years old, Kai is superior to all three.
"Voller also shares this view that none of the three at this age had such skill. He explained to me that Havertz would also be the more exceptional player in the medium term."