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Before 12.04, Ubuntu server installs a server-optimized kernel by default. Since 12.04, there is no difference in kernel between Ubuntu Desktop and Ubuntu Server since linux-image-server is merged into linux-image-generic.
For Ubuntu LTS releases before 12.04, the Ubuntu Desktop Edition only receives 3 years of support. This was increased to 5 years in Ubuntu LTS 12.04 In contrast, all Ubuntu LTS Server Edition releases are supported for 5 years.