An .xls file is a Microsoft Excel spreadsheet file containing data organised in rows and columns. A .kml file (standing for Keyhole Markup Language) stores geographic information, including points, shapes and textures. This file type is used by several programs, including Google Earth. A properly formatted .xls file can be converted to .kml format using a software program.
Use csv2kml (free). First use Microsoft Excel to save the .xls spreadsheet as a .csv (comma separated value) file. After uploading your .csv file to this program, you can select fields that describe latitude, longitude and other quantities. This means that your .xls file does not have to be in a specific format to be compatible with the program.
Use EPoint2GE (free). This program imports your .xls file and allows you to select your coordinate system and input file format before creating the .kml file. It also allows you to select custom Google Earth symbols for each of your input points.
Use GPS Visualizer (free). This web application can upload your spreadsheet, customise it, and then let you download the completed .kml file. You first must use Microsoft Excel to save the .xls spreadsheet as a .csv file. After uploading the .csv file, you can edit track and waypoint options. You can also upload and combine multiple input files into one .kml data file.