The file extension ACV marks so-called Adobe Photoshop Color Curves files, ie color-curve files of the image-editing software Photoshop. Color curves, for example, can be used to change color values such as red, green, blue or tonal values for highlights and shadows in an image.
In the German version these files are called gradation curves.
The dialog box can be reached under 'Image-> Adjustments-> Gradation curves'. In this box curves can be saved and loaded. Usually ACV files are stored under Preferences / Curves. This makes it easy to reuse useful settings.