The report comes as the European Union steps up its economic and diplomatic pressure on Belarus over mounting rights and other abuses in cases like Kavalenka's, which the European Parliament's president recently described as "appalling."
Kavalenka's wife, Alena, described him after her visit as "happy, but...very thin, half-alive."
"He says he's lost more than 30 kilograms," she said. "There is completely no fat layer left, but he will not end his hunger strike. He said he will fight to the last."
Kavalenka has been in detention since December 19 and has been on a hunger strike for most of the time since then, although prison authorities at one point tried to forcibly feed him.
On February 24, the 37-year-old was sentenced to 25 months in prison for violating the terms of his parole on an earlier conviction on charges of "illegally displaying the banned Belarusian national flag."
He has rejected the charges.
Kavalenka is a member of the Belarusian Conservative Christian Party-Belarusian Popular Front.
The expansion of EU sanctions on February 28 over democracy and rights concerns sparked tit-for-tat diplomatic expulsions that further strain relations between Minsk and Brussels, which have deteriorated since a flawed presidential election in December 2010 left Alyaksandr Lukashenka in power and sparked street demonstrations.