Sudoku XV puzzles (which are nothing to do with Sudoku-X) add two extra symbols to a regular Sudoku puzzle. All pairs of touching squares which add up to a total of 5 or 10 are marked with a 'v' or 'x' respectively. This applies to left/right, up/down touching only - there are no diagonal markers.
All possible 'v's and 'x's are shown in the puzzle, so if there is no 'v' or 'x' between two touching squares then they do not add up to 5 or 10.
This means that every time a 1 & 4 touch or a 2 & 3 touch there is a 'v' between them. And every time a 1 & 9, or 2 & 8, or 3 & 7, or 4 & 6 touch there is a 'x' between them.
Standard Sudoku rules also apply: place 1 to 9 (or 1 to whatever the width of the puzzle is) once each into every row, column and bold-lined box.