Hydronium ions (H30+) can form in a reversible chemical reaction involving water molecules.
Water (H20) is a polar molecule because the oxygen side of the molecule is negative and each hydrogen atom is positive. Because of the polar covalent bonds in water, the shared electrons spend more time near the oxygen atom compared to the hydrogen atoms which makes the v-shaped molecule polar.
This property causes molecules of water to be attracted to other molecules of water. The slightly negative oxygen in one water molecule is attracted to the slightly positive hydrogen of a different water molecule. Hydrogen bonds between molecules form and break between different water molecules and reform again.
Sometimes, a hydrogen atom in a hydrogen bond between two molecules of water shifts from one molecule and moves toward the other. When the hydrogen atom shifts, it leaves an electron behind which causes it to become a hydrogen ion (H+) which has a positive charge. The water molecule that lost its proton becomes slightly negative and is called a hydroxide ion (OH-), while the water molecule which gains the proton becomes slightly positive and is now known as a hydronium ion (H30+). This reaction is reversible because water molecules can disassociate as well as reform.
I have provided a link which shows the reversible chemical reaction I have described in the answer above.