How does open circuit voltage work?
KVL can be more generally stated as: the electric field is a conservative vector field. This can be expressed by the path integral:
\begin{align}
\int_{p1}^{p2} \vec{E} \cdot d\vec{l} = V_{p2} - V_{p1}
\end{align}
I.E. the integral is "path independent", and the result is the same no matter what path you take (so long as you start at p1 and end at p2). This can be 0, but it doesn't have to be. If \$p1=p2\$ (taking a closed path),
\begin{align}
\oint \vec{E} \cdot d\vec{l} = 0
\end{align}

This is why KVL even applies: KVL states that in any closed loop, the voltage across it is 0. It doesn't mean that the voltage must be 0 between any two points on that loop. It doesn't matter if there is any conductive path between two points. Having an ideal wire is just an extra statement that two distinct points are at the same voltage.