For a start, they're taking a train - Michał Dworczyk, the Head of the Polish Prime Minister's Office, held a press conference on the morning of the 15th in which he informed reporters that the train carrying the PMs had just crossed the Polish-Ukrainian border. This information was also released on Twitter by the Chancellery of the Prime Minister. According to the BBC's Katya Adler, this decision was taken "because flying by Polish military jet could have been viewed by Russia as dangerously provocative".
The news organisation PAP reported on the press conference held by Dworczyk, which included a question on how the leaders' security would be ensured (my translation):
Dworczyk was asked how the security of politicians going to Kiev would be ensured. "Today, hundreds, thousands of civilians in Ukraine, children, women, not to mention soldiers, are dying under Russian fire, but it is known that civilians are most affected in any war. And today everything must be done to stop the Russian aggressor. This is a risk that must be taken. In such situations, danger is inevitable " - said Dworczyk.
At the same time, he stressed that "of course, you always weigh the related risks." "Here, after an analysis not only at the Polish level, because, as you can see, this delegation is international, it was decided that such a trip must be carried out. We cannot allow the Russians to continue these criminal activities in Ukraine," he stressed.
The Mayor of Kyiv, Vitali Klitschko, announced that Ukraine's military command has imposed a curfew in the city from 8pm on the 15th until 7am on the 17th, although it's not clear that this is specifically related to this visit.
Finally, Petr Fiala's announcement of the visit on Twitter notes that international organisations including the UN have been informed of the visit - Dworczyk mentioned that the international organisations informed also included NATO and OSCE. Russia is a member of OSCE and, of course, the UN and we can probably conclude that forces around Kyiv will have been made aware of the visit, as an attack, accidental or otherwise, on three NATO heads of government on a well-publicised visit would probably be seen as escalatory.