Open KLM File
KLM extension files are of the Keyhole Markup Language (Google Earth) type, that's where their acronym comes from.
The KLM file extension is written using the Keyhole Markup Language, an XML notation for expressing geographic annotation and display on Internet-based 2D and 3D maps.
More more about Keyhole Markup Language
This is an XML-like notation suitable for expressing geographic annotations and displays within two-dimensional maps and three-dimensional land navigators.
KML was originally developed as Keyhole Earth Viewer.
The KLM file is used by dimensional Earth navigators based on Google Earth.
A KLM file contains placemarks, images, polygons, 3D models, textual descriptions, to display in Google Earth.
The compressed version of this file uses the KMZ file extension, which stands for KLm Zipped.
Specifications for this type of KLM file were submitted to the Open
Geospatial Consortium (OGC) in order to achieve open standard status as a
geographic information exchange format.
In 2008 it definitely became the new industry standard.